STEP 1: Open up Cheat engine and Call of Duty MW3 Multiplayer
Step 2: Go to your Baracks and hover cheat engine in front of mw3
Step 3: Click the little flashing green computer in the top right hand corner of cheat engine
Step 4: Scroll down on the Process list that come up to something that said iw5mp.exe it will have a little Mw3 Emblem next to it
Step 5: Look at your current EXP and type that into the Value box in cheat engine, Click first scan. Some adresses will come up on the side. Thats a good sign.
Step 6: Now go back to MW3 and go to a game and get 1 kill. This is basically making it easier for cheat engine to find the adress.
Step 7: Go to Baracks and look at the exp you have now, Type that into the Value box and click next scan. KEEP REPEATING STEP 5,6,7 until you get about 10 or less addresses, then Select all of them and click the red arrow pointing down in cheat engine. The adresses you just highlighted should be down the bottom again. Then highlight all of them and right click goto Change recordand then change value. Change the value to 1764200 this is level 80. If you want to prestige, then go back to cheat engine and click the boxes next to the adresses down that bottom, THis basically freezes them to stay on level 80.