If others can play it fine but you cannot, that alone indicates to me that it is not necessarily a Synergy issue but more than likely an issue on your specific system. Mounts are found under the 'Game List' option. If you've had it started at one point and Mounted anything under the 'Game List' option you should also verify those as well as it is attempting to load them if they are set to be mounted.
Don't opt-in to any Betas unless you are actually beta testing. If you've opted-in to any Betas opt-out of them. Synergy is dependant on HL2 so you should verify HL2 as well, not just Synergy.
