Venomous

Hello, I am new to the forums and I am in need of some help. I recently bought a Trek 3900 and I noticed that the disk brakes don't grab very good, they are in fact fairly bad. The bicycle is used. I tried adjusting the mechanism so grabs tighter, no change. I then began to get frustrated, I took out the brake pads, they looked kind of smooth. But when I began to put the brake pads back in, I noticed a small cylinder shaped magnet (this magnet was also there when I took apart the assembly). Now I can't find out where this magnet goes.

It won't sit in the center of the brake pad because the magnet repels, therefore, sliding to the side.
If I let the magnet sit on the side, the brake pad grabs only on a specific side of the disk, therefore damaging it.

Now every time I brake, the brake won't return unless I stop and roll the wheel the opposite way, or manually pull the clamp back. I even tried adjusting the spacing of the pads and all, but I was not successful.

All help is greatly appreciated!