The problem could well be that the client isn't actually running, which is why it's not found.
When you're running in UISuppression mode, you're responsible for everything which the client normally does, such as starting up, shutting down, and signing in.
You have to start the Lync client using LyncClient.BeginInitialize
- you can't just start it from the start menu. You should also call LyncClient.BeginShutdown
when you're done. (ref: MSDN)
There's a good MSDN article on UI Suppression in Lync SDK here, and there's also a channel9 video on signing into Lync when UISuppresion mode is on.