1 - Insert the VST version of Reaktor on a track. Don't use the AU version, AUs typically don't send MIDI out).
2 - Insert a AU or VST Software instrument on another track and set the MIDI in options to Reaktor
Mac OSX 10.6.2, KORE 2.10, REAKTOR 5.15.002, SAFARI 4.04, CRHOME 4.0.249.30, FIREFOX 3.6