Its easy. Here are some simple steps.
1 ) Close out of Steam.
2 ) Go to Google.com.
3 ) Search "Why Am I ..... MW3 Modded Lobby"
4 ) Click the third link.
5 ) Read The First answer.
6 ) Rage.
7 ) Go back to Google.com
8 ) Search "How to cure anger management" or "Why am I such a narb"
9 ) Read whatever you need.
10 ) Turn off your computer, and leave it for 4 weeks.
11 ) Come back, and repeat this process from step 1, and try again.
If you follow all that correctly, you should have a MW3 modded lobby for PC in no time.