You should be able to:

1) Loosen the rear bolt with an allen key
2) Turn the knurled knob at the front to raise or lower the nose of the seat
3) Tighten down the rear bolt to fine tune the levelling - but make sure its tight ENOUGH to prevent fore and aft sliding of the seat.

If you've already reefed on the knurled knob with pliers without loosing the rear, and now its spinning freely from the bolt, you need a new front bolt/knob assembly - you've stripped it.