Never mind, I got it.  Needed to re-scan existing plugins.  Why, I have no idea, but it worked.

And now my meters are all frozen ROFL.  I freakin give up

I'm also having audio problems with Session Drummer 3.  I was using SI-Drum Kit and this works fine but I changed to Session Drummer 3 as I wanted to route each part of the kit to a different track and you can't do this with SI-Drum Kit. 
 
I have loaded Session Drummer as a Soft Synth and each part of the kit is assigned to a different track.  When I play the kit within Session Drummer I can hear the output and the metres within Cakewalk show that sound is registered but after dragging and dropping a beat into Cakewalk, when I play the track I can't hear the drums.  I have all the drum tracks output to a Drum Bus and the output for the bus is the Master which is the same as same as my guitar and vocal tracks which I can hear ok. 
 
What am I doing wrong?!

See post #6 that solved the OP's problem. Likely the same issue in your case if the 'beat' is a 3rd-party MIDI file.

Thanks for responding brundlefly.  I don't think the "beat" is a 3rd party MIDI file, it's just something from Session Drummer and I've tried various other kits and beats.  I've now discovered that if I load Session Drummer as a Simple Instrument Track then it's fine but the problem is if I load it as All Synth Audio Outputs.  The Piano Roll View shows nothing but with Simple Instrument Track it shows the beats as expected.  I'm stumped. Any ideas?

The piano roll shows the data in an instrument or a MIDI track.
 
When selecting "All Synth Audio Output: Stereo", did you also select MIDI Source?
 
Without a MIDI track, the synth will still audition MIDI clips in the plug-in UI and it is possible to drag MIDI clips into an audio track but they will not play.
 
MIDI clips must be added to an instrument or MIDI track.
 
Usually when working with multi-output synths instrument tracks are not used. Instead when inserting a multi-output synth "MIDI Source" and one of the multiple Audio output options are selected. It may be helpful to check the "Synth Track Folder" option too so all the tracks for the synth are organized into a folder.

Scook, that was the problem.  I had no MIDI track!  Thanks for your help.