Haydn,
I'm also not a user of the Powermate but maybe I can help in some way. Just to be sure you installed the plugin correctly by clicking on it and it will say, plugin installed!
Now Open Mach 3 and with Mach 3 open, in the user menu: go to: Config/config plugins and the config box should come up. Now next to griffin blah blah blah, you should see a red "X" and you should click on that to change it to green check mark. Then click the yellow config box and that will bring up another box.
This is where I will be no help as I do not have one of these but have you at least completed the steps I just mentioned?
Dave