It's intentionally fairly tight -- you wouldn't want it to turn accidentally. It's also possible that sometime in the past someone might have put sealer on it. If it's completely stuck, then you'd have to get a technician to remove the top cover and unseal it with solvent. My guess, though, is that it's simply a bit "firm."
As rogue_designer said, a small spanner wrench works well. I've also had good luck with a pair of square-tipped tweezers; these are cheap and, if necessary, you can file down the tips so they fit perfectly into the slots on the prism ring.
Important tip: When you're turning the prism ring to adjust it, turn it clockwise only. The rotatable ring is fitted into the main housing of the RF unit by threading into it, and (particularly if it's a bit sticky) turning it counterclockwise could cause it to unscrew instead of just rotating. If it comes unscrewed you'll have to screw it back in, and that's a much more touchy job.
Another thing to note is that after correcting the vertical adjustment, you'll have to reset the horizontal adjustment at infinity. The reason is that the thing that controls the vertical adjustment is actually a shallow prism: as you turn it, it makes the RF image move in a circle. So, getting it in the right place vertically also moves it horizontally a bit. If you've read Karen N's page, you probably already know that the horizontal adjustment is a tiny screw hidden behind the large chrome screw at the lower corner of the viewfinder window (the chrome screw is just a cover.)
There's no reason an owner can't make these adjustments as a DIY project, assuming you have a light touch and the correct tools (the horizontal adjustment screw needs a very tiny straight-bladed screwdriver.) But if you're nervous about mechanical things or just would rather not bother chancing it, it should be an easy job for a professional technician assuming you can find one who is willing to do a simple RF adjustment rather than a complete "CLA."
