Well, if iNES tells you that it can't read your file, then your file is probably wrong. Refer to included documentation (iNES.html) for information on possible causes and how to fix them.

PS: There is a lot of crappy .nes files floating around the Net, as every pirate out there tried inserting his custom stuff all over the place. Some of them may actually load into other emulators, but iNES strictly enforces .nes format.