Im sure the cable will be fine.
First thing to do is verify exactly what ECU you have.
Then its a case of trialling some settings with the computer - I found I had to change the latency of the connection in the port settings to get mine to work reliably.
If you are having no joy with FES there are other OBDII programs out there, some are free of charge and at least give demo's - I have managed to get a few connected up to my Spider in the past. They offer way less functionality than FES, but at least you can verify that the computer can indeed talk to the ECU.
These things are always trial and error, just keep going!
-1998 916 Spider 2.0TS Lusso-
H&S Corse stage 1.
CATCAMs stage 1 cams with Savali custom ECU remap.
305mm discs with 166 4-pot callipers.
Ragazzon exhaust mid section.
Biscione Racing 2x70mm rear section.
17" Zender Siena wheels with 3mm rear spacers.
etc etc etc!