Did you download the Samsung Fascinate drivers first? I had the same problem until I installed the drivers manually.
EDIT: Yes, you will need 2 of those files, the 3rd file is actually used to recover your phone should anything go wrong. The .pit file. I'd get that one also.
The link to the Fascinate drivers is somewhere in this thread. I have actually flashed about 6 roms on my phone since yesterday so I am pretty good with the whole process now also lol.