You need to back the game up to the hard drive first using multiMAN etc. Then find your regions 3.55 custom eboot .pkg installer (basically puts the option to boot GTA IV on the menu) then copy the USRDIR folder from the backed up game location into the newly installed directory in hdd0/game/. After this, replace the common.rpf file on in the folder with the modded files on here. You should then have the mods working.
Need any more help then feel free to ask me, I'll make a detailed tutorial in the future if I have time.
~ Sam