bit file usage
I would suggest running iMPACT as a stand-alone tool outside of ISE. (From Windows Desktop: Start, Programs, "Xilinx", Accessories, iMPACT. Replace "Xilinx" with whatever you named your install folder.) It cannot successfully run synthesis again because you do not have any source code, only the bit file.
With only the bit file, it is difficult to tell much about the design, as to inputs, output, logic, etc. This was intentional to protect the intellectual property of the original designer. Sure people will tell you that they can disassemble the bit file, but its way too much work for the result you get. Even if successful it will be just a bunch of LUTs and flops hooked together with meaningless net names.
Your best bet is to request the original design files from the developer.