Another possible cause for the "file or directory not found" error when trying to run an executable is that the executable needs a specific program loader that isn't present. I've seen this with things built for "LSB" (linux standard base) - they require ld-lsb.so rather than the usual ld-linux.so to load. The message is unhelpful because it doesn't show you what was not found. If you are still stuck after using "file" as suggested above, try:
readelf -l /usr/bin/synergyc
and look for the line "Requesting program interpreter." That will tell you which loader it needs, and you can check to see if you have it.