Among the various methods of unsticking rusted nuts and bolts, one way is to soak the threads with a solution that either dissolves the rust, or at least gets enough fluid down in the threads to lubricate them enough that one can separate the metal pieces as one normally would.
There some substances sold as "penetrating oil", WD40 falls into this category. I've also seen lemon juice, hydrogen peroxide and non-diet Coka Cola and Pepsi Cola.
My great-uncle said to use witch hazel, but I never seem to have any available when I need it.
In this case you could prop the knife with the pommel up and try to get the penetrating agent of your choice seep down into the threads. You could also just dunk the pommel end into a glass with an inch os so of penetrating agent and let it sit there for while.
What ever you do, get a flat head screw driver that fits the slot snuggly and is as wide as the slot is long. This will decrease the likelyhood of you buggering up the slot while providing the maximum torque to the screw.
