You can jump a 735i. If I remember right the battery is under the rear seat like in the 740. Therefore to jump the battery you need to find a jumper junction under the hood. It should have a cover on it that says + pos in red. This is where you connect the positive cable. then connect the negative jumper cable to a ground just like on any other car.
If your battery is original (1990) you probably should replace it. Otherwise you can check the fluid level of the battery. If the water line is low the battery needs to be replaced. If you replace the battery and it keeps dying. Or you have a newer battery in it and it keeps dying after it is jumped. then you probably either have and alternator problem or a short somewhere that is draining the battery.