It is greyed out till the Radio Button Next to the word "NETWORK" is not selected. But i really dont think u have not selected the Radio Button!!
At one of the user locations, i had noticed that even after selecting NETWORK and entering the Server Name, Revit was not able to pull the License! So as a workaround, i entered the S/N, which was weirdly not greyed-out(generally it would). And then Revit could load the license properly!
So check out what works for u.