Mate this is a sure fix to this! I tried it myself and it works.
WARNING: There are two types of MW3 disks: BLES(UK) and BLUS(US). When the " downloadable content" dialouge appears and you press (Yes) It will direct you to "No content was found." Why? Because you are not in the UK and US zone. What UK and US zone? When you make your PSN account you didn't pick neither UK or US. So the solution?
1st: Check your MW3 disk case and on the side of the case look at the bottom. You will see BLESxxxxxx or BLUSxxxxx.
2nd: Make a new user in your PS3 and make a new account. And when you reach the pick your country. Check if the code in the disk case is BLES or
BLUS.Don't pick any other country its either UK or US. If BLESxxxxxx pick UK if BLUSxxxxxx pick US.
3rd: Now run your MW3 and pick a game mode and When you see the " downloadable content" dialouge pick (Yes).
4th: If you've done the steps correctly you will see 4 items in the store.
5th: Download the Terminal map or the Erosion/Aground Map.
6th: Play the game without any problems and Have fun.