If it tells you that the license has expired, it will ask also for a serial number. Try to enter the serial number received when you purchased the software and follow the instructions. If it doesn't work, set the computer clock 2 or 3 years back, launch the application and go to Help > Deactivate > Erase This Serial Number/Deactivate Permanently. After that, quit the application, reset your computer clock and, when you launch it again, enter its serial number. This should fix the problem. You can check the official web page for further information.
