When downloading your optional plugin for PowerCore, you actually download a VST plugin. In order to use this as AudioUnits, you need to convert it from VST to AudioUnits using the TCAU wrapper supplied with the PowerCore installer.

If you've already been using the PowerCore the wrapper is already insatlled.

Move the VST plugin to the folder: MacHD>Library>Audio>Plug-ins>VST>Powercore

Then Run the PowerCore software installer and choosing "Refresh Audio Units Only" from the installer's menu.