Hello, I'm asking this question because I recently found the solution to this issue, and couldnt find any info about it on the forum! Hope this post helps someone out. 
Steps for reproduction: 
- Open Ableton
 - Import 5 Reaktor AU
 - Load in a sequencer, I used Spiral from the factory library
 - Enter edit mode
 - View Ensemble connections
 - Under "Connect - MIDI Out - External" choose plugin
 - Try to set the input of another MIDI channel to the Reaktor MIDI out
 - Channel does not appear, see screenshot