Use Amon_RA recovery and flash the ROM from the SD card manually. Just follow the recommended steps in the OP from XDA and you will be fine. There are so many problems with ROM Manager and ClockWork recovery. I use Amon_RA and haven't had a single issue flashing a ROM. I also use Calkulin's format all zip file to do all the needed wiping prior to flashing the ROM. Works for me ;-)
Another thing to try: Re-download the ROM just to make sure you got a clean download file. Sometimes the get borked during download and cause all kinds of problems when installing.