So here it is, the official temporary fix for (most) DLC issues. Do note that this only works for people who don't have the DLC. If you do, stop reading.
Download iw_25.iwd and iw_26.iwd
Move the files you just downloaded to "C:\Program Files\Steam\steamapps\common\call of duty modern warfare 3\main" (note: this is "Program Files (x86)" on 64-bit systems)
Enjoy the game again!
: Downloading these files will not allow you to play the DLC without purchasing.