You can display your license key by selecting "About FileMaker Pro Advanced…" from the FileMaker Pro Advanced menu and clicking the Info button.
Pull down the Help menu and select "About FileMaker".
Click the Info button.
You'll now see your license key.
Each version requires a unique license key. If you are updating from 12 to 13, you have to have your license key. Filemaker customer service MAY be able to help. Not sure.