Where do you you insert the serial number for Team Foundation Server?
This is not answered in any of the documentation that is provided for TFS nor could I find any online forum. It certainly is not through the products supported by TFS (Visual Studio or Team Explorer).
Here is how you insert your serial number (product key) to upgrade from trial to fully licensed installation.
Go to Add/Remove Programs and select "Change/Remove".
The next screen will allow you to insert the product key.