Be sure to update to the latest version of DP4 including an MAS and/or AudioUnit updates.

If you have installed an AU plugin and it is not showing up in DP 4.1 or higher, first check that it is properly located in:
Macintosh HD/Library/Audio/Plugins/Components.

If it is not in this location, reinstall the AU plugin.

If it is in the Components Folder and DP is still not seeing it, check that the Audio Unit Support Bundle is located in:
Macintosh HD/Library/Audio/Plugins/MAS.

If it is not in this location, reinstall the latest MAS plugins located here: www.motu.com

If you are still experiencing problems, you can force DP to examine AU plugins by trashing the Audio Unit Info Cache located in your
Macintosh HD/Users/Login/Library/Preferences/Digital Performer.
Next time you launch DP it will examine your AU plugins to determine which ones will be stable in DP.

If DP passes the AU you are trying to load you should find it as an insert in the mixing board or if it is a soft synth under the Project Menu > Add Track > Add Instrument Track.

If your plugin is not passed by DP, report this to MOTU and the maker of the plugin.

A report on which AUs are passed on your system can be found in your User's Documents folder called, "MOTU AU Test Results."