If you searched before, you'd have seen this was already answered here.
Here's the part you're reading:
"I got a message : "Files different from server"" : There is two problems that make this message, the first is that you got the "wrong update"
By wrong update I mean that the normal "22.214.171.124" and the "126.96.36.199-DLC maps" aren't compatible. I advise you to make a copy of your MW3 folder and put the DLC maps in one of them.
Even if you tried DLC / No-DLC game and you have this message on a server, well, I don't know why does it display it, I also have error message on some servers.
/!\If you want to know how to DLCs, use Google./!\