Well I'm guessing you're italian so your english is a bit off. Just for clarification sake there's no such thing as Official Sony Custom Firmware Kmeaw 3.55, That would be an oxymoron. Yet again I assume you're are not an idiot just not fluent in english. There's sony OFW (sony official firmware) that doesn't allow jailbreak. Then there's CFW (Custom firmware) like Kmeaw 3.55 you mentioned that allows jailbreak.
Anyway to install a patch, you download the patch (a .pkg file with a long filename) then put it on anything usb in the root folder (external usb hard drive, usb thumb drive, etc). Put the game on external. Then plug the usb device in and go to install package files on the XMB menu pick the .pkg file it will install. Then you can run the game and it should be patched.
This game works sometimes without the patch, but for some people it wont work so then try with the patch. Also, be sure game is on external w/ BD mirror enabled, some claim it runs discless but didnt for me. If it still doesnt work create ASSASSIN_ISO folder in USRDIR directory that worked for me as last resort.
PS3 Slim 4.21.2 Rebug REX 4.31 Spoof (external 1TB/3TB), PS2 Fat w/ Internal HD