what you do is after you beat the final mission (on regular or higher), you wait for the credits to end, and you should play the epilogue. on an airplane. it gives you 2 minutes to beat it. once you beat it, it says arcade mode, and cheats unlocked. then all you do is collect all the intel to unlock certain cheats. once you have all of them. during campaign, just pause>options>cheats and select the cheats you want activated
here's the intel guide:
Call of Duty 4: Modern Warfare Laptop Locations FAQ - IGN FAQs