This is the question I hear a lot.
People install a TFS trial and when they license it with a volume license agreement but they can't find their serial number to upgrade from trial to licensed version.
After failing to find the key, they do the sensible thing. Call Support. :-) Which normally doesn't help.
Here is the recipe to quick found your key.
Download the TFS ISO file from the volume licensing site.
Either burn the image or use an ISO reader to see it's contents.
There should be a folder called AT which contains a file called setup.sdb
Open setup.sdb with a text editor (notepad will do fine) and search for the string Product Key
The next line contains the product key.
Voilá. You are now read to fire up TFS setup (control panel->add/remove programs) and enter your serial key.