Before you join us in chat please read through this how to from top to bottom at least once.  Thanks.

If you need help doing this click here to join us in chat

Ok if your running windows we have an easier method for you here if that doesn't work for you proceed with the steps below.

Introduction:
This walkthrough will take you through rooting a stock 1.5 Eris. After following the walkthrough, you will have a rooted 2.1 Eris.
Unfortunately, this will NOT help people who upgraded to the Leaked 2.1 ROM. For now, there is no way to fix your issue.

CREDITS:
GrdLock for his walkthrough in androidforums.
jcase for finding the rooted dev ROM
amon_ra for the recovery console
ivanmmj for rooting the 2.1 ROM

Warning:
Neither I, nor thefuzz4.net or xda-developers.com, is responsible if this process bricks your phone. Sometimes, the unforeseeable will happen. Sometimes, you will make a mistake. Although this process is relatively safe, if your phone becomes bricked, we do not and will not provide support or replacements for your bricked phone.

Index:
I: Gaining root: an unlocked hboot
II: Prerequisites: Tools to flash
III: Flashing a rooted/modded recovery
IV: Flashing a custom ROM (rooted version of leaked 2.1 ROM)


I: Gaining root
An unlocked hboot will allows us to flash in a modified IMG ROM signed with test keys as supposed to requiring an official ROM from HTC. This will open us to be able to flash a modified recovery.

1. Download this file PB00IMG.zip Eris Root Rom from down below.

2. Place PB00IMG.zip onto the root directory of your sdcard.

3. Power down your phone.

4. Hold Volume Down while you Power On. This should bring you to a screen saying "HBOOT" and some other stuff. Wait for it to load the image, and it will say Push Activate. Push the trackball button to continue. The process will take around 5-10 minutes. The first time your phone boots up it will take a lot longer than normal.



II: Prerequisites - Android SDK - These instructions are for Windows XP. You must adapt them to your version of Windows.
First we must install the Android SDK as it has the tools we need for both this walkthrough as well as to make your life in the Android world easier.

1. Visit the Android SDK site.

2. Visit the Java site and download the latest Java JDK.

3. Extract the Android SDK zip file unto c:\AndroidSDK\

4. Double click on SDK Setup.exe

5. Many times, you will need to click on Settings and check the box to force HTTP instead of HTTPS.

6. Choose to download the USB drivers package. Nothing else is required.

7. Enable your phone's USB debugging mode: Settings > Applications > Development > check USB Debugging

8. Connect your phone via USB to your PC.

9. Right-click My Computer > Properties, click on Hardware, and go to Device Manager. You should see a device with a yellow exclamation next to it, this is your phone (if you don't see it you probably forgot to enable USB debugging mode)

10. Right-click the device, select Update Driver. Browse to your Android SDK directory, and you should see a "USB Driver" folder in it now. If you don't, you forgot to download the USB Driver package when in "SDK Setup.exe". Select the driver in that folder.

11. To make life easier, lets add a global path to the android SDK tools. Right-click My Computer > Properties, click on Advanced, then click on Environment Variables. Doubleclick on PATH, then at the end of the Variable value, add a semi colon ";" and then c:\AndroidSDK\tools



III: Flashing a Custom Recovery
Flashing a custom recovery will make your life easier when flashing ROMs in the future.

1. Download the Recovery file from down below.

2. Move the recovery.img file to your \tools\ directory of your Android SDK.

3. Open up a command prompt and go to your tools directory, and execute these commands:

         WINDOWS USERS

        From your tools directory in the sdk do the following command

       adb reboot bootloader

        You will have FASTBOOT highlighted in red like it is here

        Now on your computer do the following

        Fastboot flash recovery recovery-RA-eris-v1.6.2.img

        Ok after that completes pull the battery then power back up with Vol Up + Power to enter recovery

        Windows users continue on with step 4

        LINUX/MAC USERS

       You need to download the recovery.zip file from down below

       (if your adb is not working right make sure you chmod +x adb)

• adb shell mount -o rw,remount /dev/block/mtdblock3 /system
• adb push  recovery.img /sdcard
• adb push flash_image /system/bin
• adb shell chmod 755 /system/bin/flash_image
• adb shell flash_image recovery /sdcard/recovery.img

4. Success! You now have a recovery partition installed on your phone. You can access it by running "adb shell reboot recovery" or by powering off your phone, holding "Volume Up + Power On"

IV: Flashing a custom ROM

1.  Download the custom rom of your choice from here.

2. Copy the WHOLE zip file to the root directory of your SD card. (You can enable USB-MS enable from your shiny new recovery to mount your phone as a USB Mass Storage device.)

3. Choose Wipe. Choose data/factory Reset (In the stock ROM, the dalvik is stored in the data and you don't have an EXT partition... yet.)

4.  Choose Flash zip from sdcard. Find the zip file that you downloaded earlier.

5. Wait until it shows as complete and choose reboot system now.

CONGRATULATIONS! You now have a rooted 2.1 phone!

Now then get all of your applications installed get your phone all setup and working the way you want it.  Once you are done go here for the how to on making a back up of your phone

If you are in Windows XP there are drivers down below for 2.1 that you will need to install and once again this is only for Windows XP.  Thanks.

 BTW if you are looking for change logs please check the forums.  Thanks