No matter what I do - and I've followed the Help instructions to the letter! - AudioSnap won't actually turn on. The 'pallette' comes up, I have a clip selected, I've put the track into 'Transients' mode, but none of the buttons in the pallette function, mainly because the on/off button won't light up.  What am I missing? 

Make sure the clip you want to convert to an AudioSnap clip is a regular audio clip, and not a Groove clip or Region FX clip.
 
As a test, make a copy of the clip, right-click the copy and select Bounce to Clip(s), then try to enable AudioSnap.

Thanks Morten!