I've downloaded PowerMate 3.0, but Mountain Lion says it can't be opened!
Mountain Lion includes a security feature called Gatekeeper, which prevents certain applications downloaded from the internet from being installed or opened. To bypass Gatekeeper so that you can use PowerMate, find PowerMate in your Applications folder, then hold the Control key and click on the PowerMate icon. Select "Open" from the menu that appears. You'll get a message from Mountain Lion asking you to verify that you want to open PowerMate. Choose "Open". You may have to enter your password at this point. PowerMate will now open normally.
You'll only have to complete this process once. This will add PowerMate to Gatekeeper's exceptions list, and you can now open PowerMate just like any other application.