There has been a bit of discussion on the IC-7600 newsgroup about using FSK vs. AFSK when connecting a computer to the radio. Here is a summary that I posted in that group. Hopefully it will help some others get a kick-start on the process:
I found what I think is a good summary of the options for using the 7600 on RTTY. Since I am not sure if it is appropriate to repost it here in its entirety, here is a link:
Bottom-line, you can use the radio in AFSK or FSK mode, but the interfaces differ (as others have mentioned here). If you want true FSK using an external computer, you need a cable to provide the keying input to the rig. Since that allows you to use the great filters in the radio, that is a good idea. But for getting started quickly, all you need is a USB interface and some software like MMTTY, to run in AFSK (which works quite well for me using an XP PC).
For verifying your USB drivers work, why not just install one of the basic rig-control programs and confirm that you can tune and access various functions from the computer?