The rings are located directly behind the yoke. They're in pairs, and different adjustments are done by either rotating them as pairs, or by adjusting the relative position of two rings within a pair. It's complicated, and kind of hard to explain, but I would suggest reading the manual for the Electrohome G07 or one of the Wells Gardner manuals - they go over the convergence procedure and what all the rings do.
Adjusting the convergence on a Nintendo game is a pain, because they don't have a built in crosshatch pattern. At least, the classic ones don't, I do not know about the PlayChoice. At least with the PlayChoice you can get a menu with static white text on it.
If the convergence is good in the middle and only bad in the corners, then you might not need to adjust the rings. The position of the yoke also affects convergence, and if it's crooked because of a missing rubber wedge, then the sides would be off but the middle still good.
How bad is the misconvergence, and do you have a picture of what it's doing? Some things might not actually be a convergence problem - it might just need to be degaussed. There is also two separate adjusments - convergence and purity. The purity rings are the pair closest to the yoke, and they adjust to make sure the red gun only hits red phosphors, etc. If the purity is off (or the tube needs to be degaussed), you will get discolored blotches - but the convergence would still be OK.
Be warned that convergence is a really tricky process. It will take you a long time, and it is easy to screw it up even worse. Mark the position of the rings with a Sharpie or something before you move them around, so at least you can get back close to where you started.
-Ian