First, I would stay away from ROM Manager if I were you, at least until you are a lot more familiar with how to use Amon's Recovery, verify that a downloaded ROM file is complete, and know how to restart Clockwork Recovery from Amon's.
For any potential rooter who has not done this before who is reading this, please do not use the 1-click root method - it is not very well documented, or at least not nearly as well as the Universal Root for Dummies method is.
For instructions on how to install ROMs, read the Universal Root for Dummies thread, starting here, at the fourth post:
http://androidforums.com/eris-all-things-root/127861-universal-eris-root-dummies.html#post1186824
Ok, enough with the lectures.
If you truly did back things up, you should be able to restart from Recovery by powering down, then holding VolUp while you power up. From there, go to the Flash a file menu, and there should be a file called update.zip - flash that and it will start Clockwork Recovery.
From Clockwork, go to the Backup and Restore menu, then Restore, and, if you really did back up, your backup will be there, ready to restore.