Did you install the Samsung drivers, and is your phone in debug mode?
I would go over the procedure once more to make sure you didn't miss anything. After that try a different USB port and/or USB cord.
To go back to 2.1, refer to this thread.
EDIT: Just noticed your comment about Odin. This is probably the simplest form of going back to 2.1 at this time, so I strongly suggest it. If you're confused about anything in the directions definitely feel free to ask.