I sent your heatsink/sleeve/o-ring today. (Thursday)
The host itself is ground. (battery negative to tail cap switch)
You need to get the negative input lead to the driver to touch the host.
I showed one way to do it, but you could also use the diode case pin if you wanted to. In this case, you would not need the little wire I showed. Just your positive lead to FlexDrive. And the negative would come from the diode.
As long as the heatsink is touching bare metal. If not, then you just file off the anodized area where the heatsink would be touching when tightened, and then you could use the diode case pin to be the 'ground to driver'.
But the Mini Mag has anodized threads at the head, so this may be a problem for getting good ground from the host to heatsink.
A FlexDrive only needs one ground connection. Most of the time, I like to give ground from the host or pill, and then connect the positive and negative driver outputs to the diode. Because there is less chance of a bad connection, which could kill your diode.
But if you have a good connection from your heatsink to your host, then it is fine to get the ground from the diode case pin.
With a red build, the case pin is the negative. With a blu-ray build, you would just solder the case pin to the negative pin, and then the negaive wire to your FlexDrive...
See the 4th wiring diagram in the FlexDrive documentation:
http://hacylon.case.edu/ebay/laser_diode/MicroFlexDrive_V5_manual.pdf