Product Key is Invalid Error in EndNote Mac
It's possible to get on the incorrect installation track when installing EndNote on a Mac, which will result in a "Product Key is Invalid" error. The following screenshots are from version X7, but it's been possible from previous versions, and will probably be possible in future versions.
At this point, you should drag the EndNote icon as indicated, locate EndNote in the Finder on your Mac, and run it. If you ignore the instructions and double-click the EndNote X7 icon, thereby launching EndNote from the mounted disk image, you will see this:
If you ignore that warning, click "Continue," and enter your key, you will see this:
The solution is to cancel out of the application, drag the EndNote icon to the Applications folder, and then launch it from the Finder.