You shouldn't use Rom Manager for much other than backing up. There have been a bunch of cases of people flashing using Rom Manager & the Rom not working correctly, just an fyi
Does anyone know why das bamf has a category but no roms in Rom manager ???
There's a sticked post written by Cyber Warrior in the Thunderbolt rooting subforum that provides a great step-by-step explanation of how to use Clockworkmod to backup and flash ROMs. It's preferable to use Clockworkmod to flash ROMs, rather than ROM Manager.