I note from the docs this mic comes standard with a 4-pin connector, not too dissimilar to mini-XLR. However the one handed to me has 3pin mini-XLR so I tried it with an AKG B29L (http://www.akg.com/site/products/powerslave,id,42,pid,42,nodeid,2,_language,EN.html) mixer/power supply but get nothing. The B29L provides 9v phantom power, the X391 power requirements are listed as: "11 to 52 Vdc phantom".

Should it work? In the box was what I assume to be a Shure pre-amp, black cylindrical with the above mentioned 4pin mini plug to XLR- 3pin full size. Can't use that to connect to my camera because of this wierd 4-pin connector.

Where did you obtain this equipment? Could some of the items have been mixed up? The MX391 is supposed to have a 4-pin miniature connector, but the very similar MX393 is listed as having a 3-pin miniature connector and its preamp is built-in, therefore it uses a regular miniature 3-pin to regular 3-pin XLR cable.
Was the AKG mixer/preamp part of this kit of gear or did you just substitute it in because it had a connector that fit?
Give us more details, as well as the camera you're connecting to.

It was a "here you might be able to use this in the video prod unit" handover from another area - presumably AKG added by someone else in our organisation. The pre-amp I think is part of the X391/O kit. Using Sony HVR-Z5P camera.

The Shure docs mention attaching a different connector which seems to have been done so as to plug into the B29L. Seems I need a min-XLR to XLR to use the boundary mic on the camera and just ditch the B29L.

Well, if your mic requires 11V and you can only supply 9V, there's no reason to expect it to work.

I guess I'd had expectations because presumably someone along the line had put this together thinking it would work - I'm not au fait with all these voltages 9, 12 40, 48v I didn't know there were so many current/voltages variances mic work at.

I think the AKG mixer is neat little unit - I'm looking to get a couple of self-powered boundary mics that will work with it b/c I don't see the 9v battery lasting long driving two mics.