Instructions
Connect your PSP to a computer via its USB data cable.
Click "Computer," then "PSP" from the "Removable Devices."
Double-click the "seplugins" folder, then the "cwcheats" folder. Right-click "cheat.db" and select "Open With," then "Notepad."
Scroll down Notepad to "ULUS-MHF2." Below "ULUS-MHF2" you'll find the "_CO" line. Type your cheat code in front of the "_CO" so that "cheatcode_CO" is your cheat code. Press "Ctrl" and "S" to save the changes, then close Notepad.
Disconnect your PSP from the computer. Put the "Monster Hunter Freedom 2" disc into the system and wait for the game to start. The cheat code's feature should be available.