Yeah, NtAuthority did a great job again
He made MW3 private server
Great Job NTA
What you need
-Modern Warfare 3 Multiplayer files ( just torrent them we don't provide links )
-Make an account here and download the necessarily files
What you need to do
-Get the files from AlterMw3 site
-Place them in your Mw3 folder
-Run altermw3 let it update
-Run Iw5mp.exe and have fun
Remember it is still in beta state, so if something isn't working properly it could be normal.
Forum for this version of MW3: here