I had Synergy running on both a Mac and PC a while back.

I did have issues where the Mac didn't want to control the PC, but the PC could control the Mac just fine.
Are you trying to run them over a WAN or a LAN? I was running mine over a basic home network. The Mac is a first gen 20inch Intel iMac.

I have since then upgraded my PC so I've switch to keyclone and 3 boxxing on one machine.