[UF ShadowTeddy]

You don't even need a CD key to install it, as it doesn't use GFWL anymore to query the servers. So, if you select activate later, Download and place the MF.dll file from the github website in to Halo 2's install dir, then run the launcher and let it update, you should be able to play and select the network tab ingame

You Must Use The Network Tab In Order To PLAY WITH OTHER PLAYERS.