LOL if your got the 400 new weapons one all you got to do is extracted it to your desktop with zip genius of winrar. Then open the file and go AWP then go system then go AWP and you must have apocalypse weekend or it with give you a eroor message.
If its just AWP then like jojojoj92 said you gotta have apocalypse weekend expansion pack.
and postal 2 share the pain. Cheers