What version and sub version of LPX and OS X are you running?
This sort of issue was resolved in an earlier version of LPX for most people.
However, note.. make sure you have correctly run the Waves AU Reg Utility to ensure OS X's Audio stuff can properly use and see the Waves plugins.. (and don't just manually copy them across to the compenents folder but use the proper installer to install them) and make sure your Waves plugins are fully up to date....