How do I run the 2.7.1 trial version on Mountain Lion if it says it's corrupted?
The specific issue on Mountain Lion is with GateKeeper. The old trial download was not signed with the new Mac Developer ID certificate.
You have to first disable GateKeeper in your System's Security & Privacy preferences. Select the General tab, and set "Allow applications downloaded from" to "Anywhere". Once you complete the download and install, then you can set it back to normal security to allow only App Store downloads.
This compatibility issue will be fixed when we can provide 3.0 trial downloads later this year.