I'm not at home, so I don't have the software in front of me, but...
I think there's a setting to determine which kind of IMP file you are making. The software makes files for the old Rocketbook (RB), the GEB/RCA/EB1150 (IMP), and the 1200 (ETI-1) (IMP). They are all different and not interchangeable.
When I download books from Fictionwise, I have to choose the "Half-VGA" IMP file. See if you can find a preference to set the kind of output file you are creating.