If I remember correctly that is the phone I had trouble finding the battery on. You can't take the back off. There is a "door" on the right side of the phone on the bottom half (if looking at the back) where you can get the battery out. I may be thinking about the wrong phone though.
Anyways, hope that helps and welcome to the forum. Also, you may want to check out our HTC EVO section for more specific help.