I...
* click the toolbar install link
* allow the pmog.com site
* re-click the install link
* allow the certificate (why aren't they signing their plugin on something this public?)
At this point, Firefox pops up the Add-Ons window with a new installation tab. The PMOG plugin is the onl item in this tab, but it is greyed out, as is the restart button at the bottom of the Firefox window.
I'm running Firefox 2.0.0.12 on an Intel Mac. I have a couple of extensions: FlashBlock, FireBug, WebDeveloper.