I just heard about this today, I downloaded a this (viewtopic.php?f=7&t=2040) and extracted the files to my directory in steam (C:\Program Files\Steam\steamapps\common\call of duty modern warfare 2). I ran the LaunchIW4M, it download it. I logged in and i chose "browse servers". HERE COMES IN THE ISSUE.... Every time I join a populated server and ERROR appears, for example: mp_complex.ff or mp_???.ff

 

Why does this happen? 1 more thing: when i join an empty server, it works fine.

Nope. The error was because you tried to enter a server wich was running a DLC map, and you don't have the DLCs on your Steam installation (that's why you're missing mp_complex.ff, among all other DLC fastfiles)

All I can tell you is that you need to get the DLCs...

ONE MORE THING!!!! when i completed my first map (6 hackers, 2 people got banned)I got mp_crash.ff what does this mean? (i was on afghan)

ONE MORE THING!!!! when i completed my first map (6 hackers, 2 people got banned)I got mp_crash.ff what does this mean? (i was on afghan)

The server then switched to "Crash", wich is also a DLC map

ONE MORE THING!!!! when i completed my first map (6 hackers, 2 people got banned)I got mp_crash.ff what does this mean? (i was on afghan)

The server then switched to "Crash", wich is also a DLC map

Do i have to buy the dlc LEGIT from the steam store or is there a FDO special dlc download?

ONE MORE THING!!!! when i completed my first map (6 hackers, 2 people got banned)I got mp_crash.ff what does this mean? (i was on afghan)

The server then switched to "Crash", wich is also a DLC map

Do i have to buy the dlc LEGIT from the steam store or is there a FDO special dlc download?

There's no "4D1 special dlc downloads", although any DLC files pack will work. It's up to you to buy them or not. Law and Activision say that you have to pay for it. Anyway, it's your choice.

Choose one of these 2 places to get the DLC files:

http://store.steampowered.com/dlc/10180/

http://lmgtfy.com/?q=alteriwnet+dlc+repository

You only need "zone\dlc" folder of each DLC package, by the way.