The ball is just a little check valve that allows oil to flow into the joint, and prevents it from flowing back out when you remove the grease gun. Your grease gun does not have to have any special attachments. When you squeeze the handle, the pressure in the gun builds up and forces the grease past the little ball. When you release the handle, the pressure in the grease gun drops, and the little ball seats itself to prevent the grease from coming out of the joint.
That being said, if you are having trouble greasing a joint (can't pump the handle because the grease is not going into the joint), the ball in the grease nipple might be seized in position. Take something pointy and see if the ball can be pushed in (it should move freely with very little force.) If it's seized, you may have to replace the nipple.
