How do you activate cheats with letters in saints row 2 on xbox 360

I'm not sure what you mean by letters, but you can activate cheats by going to your cell phone in the menu and calling numbers like #909.

Here's a list of Saint's Row 2 cheats for the PC, I'm pretty sure they're the same for Xbox 360.

http://www.cheatcc.com/PC/saintsrow2cheatscodes.HTML