After you insert your Serial Number, you should see a request "Do you want to activate the scenery on this PC ?"
If you don't see such request, and FS9 goes back to normal flight without asking anything, it can only means you selected the wrong product from the Addon Manager, or you have inserted something that doesn't have anything to do with your Serial Number, like your Order Number or your SKU number for example.
Your Serial Number is a long code that usually starts with FSDTxxxxx-xxxxx-xxxxx-xxxx-xxxx, it's not your Order Number and it's not your SKU Number. The Serial Number is listed on your purchase receipt as "Serial Number".