If your serial number says it is not valid when you try to add, make sure that you use it in the appropriate application. If you have a MovieRecorder license for example, it won't work in MovieRecorder Express.
If you try to enter a serial number, an activation or deactivation code in the License window or on the manual activation/deactivation web page and it is refused, you can check the following common errors (or even better do a copy/paste):
- 0 is very similar to O. The first is the number zero, and the second is the capital letter o.
- I looks the same as l. The first is a capital letter i and the second is the lower-case letter L.
- You have entered an empty space before or after the serial number. To make sure, just use cmd + A keyboard shortcut to select all the text and start typing again.