The above screen shot is proof of concept that it is possible to use AudiUnit plugins in Cubase and Nuendo.
Cubase and Nuendo are themselves only willing to use VST plugins. In comes Plogue Bidule to the rescue!
Bidule is a patching environment (somewhat similar to Max or Reaktor). In addition to the stand-alone application, Bidule also comes in the form of a VST and AudioUnit plugin.
In the Nuendo project above, a Bidule VST plugin has been added. Within the Bidule patch two AudioUnit plugins are hosted, a 4-band compressor courtesy of Apple, and the MasterNeedle plugin from the now discontinued Hipno plugins. There is also an AudioNetSend plugin thrown in for good measure, as I first started investigating this when I wondered about how to get sound across from Nuendo to Spectre for advanced spectral monitoring.