According to Gamesradar.com, to get Infinite ammo you need to get all 30 intel in the game. To find all the intel, i suggest you type in "Enemy Intel for Call of Duty 4" on youtube and just do what they tell you.

There are also some other hints about the game at this URL.

http://www.gamesradar.com/wii/call-of-duty-modern-warfare-reflex-edition/cheats-guides-faqs/g-20090805145420379077