I am being asked to re-enter the serial number and I am getting the message that the activation failed.
Activation Error #1025: Either a previous license file exists or ...
Go to the global Macintosh HD > Library > Preferences folder and delete the Fontlab Studio license file "com.fontlab.fs5m.lic"
If you see the file "com.fontlab.licenses.plist" there then you are in the wrong folder. You need the global Macintosh HD>Library>Preferences folder not the user's one.
Deleting "com.fontlab.fs5m.lic" will allow Fontlab Studio to write a new license file there at the moment of activation. So run Fontlab Studio again and activate it with the serial number. If it fails again with the same error then run Disk Utility and choose Repair Disk Permissions on the startup disk.
The file "com.fontlab.licenses.plist" keeps licenses for old Fontlab apps including FLS 5.0 and Fontographer 4. It is located in the user's ~/Library/Preferences/ folder (your-user-name/Library/Preferences).