You need the amulet with the painted paw to ues the lever and not the keys, as the keys are for teh doors.
In the level with the lever, you need to battle the undead army, then go to all the side rooms. A plain key can be found to open the locked door at the bottom right of the main hall, and a skeletal body will be found in the chamber beyond the locked door. The body contains what you need: the amulet with the painted paw.
By the way, the amulet can be worn. It gives a +1 armour class, +10 hit points and some saving throw bonuses. Most importantly you must have it to use the lever.
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