I'm using a Leopold FC660C, sharing it through both Synergy software and a KM switch hardware with 3 computers: a Mac running OS X, a Mac running Windows 7 via Bootcamp, and a PC running Linux. No problems whether the keyboard is on Windows via Synergy or directly connected using the KM switch. However, I have not tried Windows 8.
I presume you have tried connecting and reconnecting, allowing drivers to install, etc. On my Mac, the FC660C is recognized as "mini keyboard", whereas my IBM SSK is recognized as "input device". It is the same on my Win 7 machine.