Hi Ian,
I have the same problem and have had for a long time!
BPM has been pretty much broken ever since MOTU released the AAX version.
I actually don't think it has ever worked in any version of Pro Tools, just causing PT to instantly crash after inserting it to a track.
Also, it's been ages since MOTU last updated the plugin and who knows if they ever will again...
My solution, and probably also your best bet, is to use the Blue Cat's PatchWork Synth plugin on an instrument track to host the VST version of BPM instead of AAX.
The VST version works perfectly this way, so there is definitely something wrong with the AAX version of BPM.
A shame really, because BPM is really one of the finest VI plugins out there!
If you have a little spare money i suggest you invest in the Blue Cat's PatchWork plugin for this little workaround!
Best regards,
Swedish