What Kickstart and OS versions are you running on your A600? The exe file name mountpc4ks3 is probably an abbreviation of 'Mount PC for Kickstart 3', so if you are running Workbench 2.05 (standard with an A600) it probably has missing files.
I have the Weird Science Network PC kit, which I imagine is similar, and I understood that this was originally only sold for A1200 or A4000 (which had kickstart/workbench 3.x as standard).
Where are you running mountpc4ks3.exe from? If you boot to workbench and then run the programme from another floppy, or a CD, is there an install routine you need to run first, which will copy the right files to your workbench disk, or HD?
As I recall on the Weird Science version, there was a programme on the CD which made a new Amiga boot floppy disk, so it may be worth running that and booting from it.