No one has responded to this yet, so I'll throw out what little I know to maybe get someone started with a "that's not right - it should be . . . "
I have the 225/40/18 tires on the front of my 330ci and the book calls for a max pressure of 32 lbs. at normal loading (4 people on board) and up to 39 lbs. with full load (five people plus luggage.)
Of course, the rears on mine are 255/35/18 and recommendations there are 38 to 46 lbs, dependent on load.
If the 225/40/18s were not an option on the 318 (and I don't think they were), then the recommended pressures are not going to be listed in the owner's manual. It should be a little lighter car with different weight distribution and handling so pressures from the 330 will not necessarily work. You are either going to have to experiment a lot, maybe call Tire Rack for an initial pressure setting, or just keep posting your question on different BMW forums until someone with a 318 and similar setup gives you an idea - of course he or she will have probably guessed, too.