I use cwcheat.
http://cwcheat.consoleworld.org/ I believe is the site. it took me a while to get used to set it up, and its been so long since Ive done it but the site has a walkthrough.
basically set it up how they say. some games dont have cheats but killzone liberation for sure does, it did at least 2 years ago. so after putting it on your psp and installing it how they say (make sure to go to the recovery menu and enable it), you just hold select for five seconds during the game to access the cheat menu