Easy Install (Modloader):
Copy the ProStreet_Assets folder into the modloader folder.

Manual install:
Copy the data folder into your game root folder - add the files into the gta3.img file - add these lines to your gta_vc.dat:

IDE DATA\MAPS\prostreetasset\prostreetasset.IDE

COLFILE 0 DATA\MAPS\prostreetasset\prostreetasset.COL

IPL DATA\MAPS\prostreetasset\prostreetasset.IPL