Heres how i get Gameshark 2 v4.1 to work.I have a mooded ps2.
1. Load GS depending what boot method your chip uses to boot a ps2 cd-based or ps2 dvd game and use that mode. 2. Select game and cheats you like to use and then press x and tray will open 3. Swap out GS disc and put in your HDA disc and press x and tray will close. 4. Wait for HDA game list to come up and choose the game you have the cheats activated for and press x and game should load with cheats. 5. Then enjoy!
In the gameshark disk the SLUS,SCES or SCUS is your elf file and should be around 4,000kbs or more in size,you can use programs like PS2OS from the PS2OS release thread in this forum to put elf files on to mc or hdd.And rename it like Bootleg Greg has mentioned,thanks was just ready to post that.