You can call Air China to add in your MP number on the reservation. Reconfirm at check-in, which you'll need to do as a good idea.

Since the trip was booked through a travel agent, I am assuming that this is all under 1 PNR, regardless of number of ticket numbers? If they are booked on separate reservations, you'll need to claim your bag at LAX and recheck with CA. If they are all on the same PNR, DL will be able to check the bags through.