If the lock hasn't been removed for a long time, the best procedure to use is as follows:
1. Using a large screwdriver that fits the two screws on the left hand side of the rifle, loosen them about 2 full turns.
2. Place a small block of wood on the heads to protect them and then lightly tap the wood, driving the screw heads back against the stock.
3. Loosen the screws another turn or two and repeat step 2.
4. Remove the screws from the lock and stock.
5. Gently push back and forth sideways on the hammer and the lock will come out of the stock.
At this point I suspect that at least one piece of the mainspring will fall out of the mortice.
If it doesn't, perhaps the mainspring is just hung up. By cocking the hammer you should see a spur on the front of the tumbler towards the mainspring rising. It should be moving the tip of the mainspring upwards.
To relese the sear from the tumbler and allow the hammer to fall, push upward on the sear arm that is sticking outward from the lock.
Removing or installing a mainspring is best done with a "Mainspring vise".
This is a small block of steel with a movable screw actuated block that is made for applying pressure at the right points on a mainspring.
These are available from the same folks who carry muzzleloading gun parts so if you call Deer Creek, be sure to ask if they also have the vise for sale.
