Is the bolt actually moving forward when you turn the nut? If not you might need to hold it with some pliers on the inside and push it so the thread will engage. Mind you everyone will tell you to be careful with the adjustment and not to overdo it because you risk over tightening and ripping the leather. You typically only need to do it to older saddles that have sagged a bit.
The amount of tension should be enough to support your weight without sagging excessively; I recently bought a second hand B73 which sagged so low that the underside of the top touched the rails when I went over bumps. Tightening it a bit gave it back its shape and actually suspended my bum.