Try the following:
Mac
    Open Finder
    From the "Go" menu select "Go To Folder..."
    Paste in "~/Library/Application Support/Syntorial"
    Delete the "UsersMaster.xml" file.
    Open Syntorial, and it should now let you authorize
PC
    Enable hidden files by going into your Control Panel, clicking Appearance and Personalization, and then clicking Folder Options. Click the View tab. Under Advanced settings, click Show hidden files and folders, and then click OK.
    Go to C:\Users[username]\AppData\Roaming\Syntorial
    Delete the UsersMaster.xml file.
    Open Syntorial, and it should now let you authorize