Doing the following activates it using the MAK key:

1) Open a cmd shell as Administrator.
2) Type the following (but use your key instead of all the X’s)

slmgr -ipk serial
3) The prompt will return right away, but there is a licensing task that it started (if you look in Task Manager you can find a task SLsvc using up cpu, I assume this is where the licensing is going on).
4) After a while a dialog will pop up saying that it has been activated.