Nuendo 5.5.1 can run at 32 bit or 64 bit on Mac OS 10.6 and 10.7.
The VST plug-ins need a 64 bit version to be loaded by Nuendo when it’s running at 64 bit. AFAIK, Waves does not have a 64 bit version of the VST WaveShell on Mac OS.
The 64 bit VST offer on the Mac is a bit sparse… You’re better off running Nuendo in 32 bit. That being said, the gain in CPU efficiency at 64 bit is around 10% and I use Nuendo 5.5.1 (32 and 64 bit) in Mac OS 10.7.1 without any problems…