Before doing this make sure that Reaktor standalone starts without any messages or warning etc. This is to make sure that setup and authorization of Reaktor is complete.

You must have installed the MAS/Audio Unit 4.12 update from MOTU to use Reaktor. Also Reaktor update 4.1.2 or later is required.

In your userfolder/library/preferences/Digital Performer find the file

Audio Unit Info Cache

trash this. Restart Digital Performer. All Audio Units will be rescanned. After this Reaktor AU should work as expected.