there should be a small adjusting screw on the back of the instrument or maybe on the face to adjust the cam to centre the needle.
If the instrument is on a desk, there will be the same atmospheric pressure outside the capsule as inside it, so the needle should be at zero.
The instrument works on the principle that a change in altitude will cause a change in pressure. The pressure around the capsule is allowed to leak into it at a controlled rate to show as a change in barometric pressure, hence a change in altitude.