Lots of good advice. Ok, I took the side plate off and the cylinder latch is supposed to fit into a small hole in the frame and push the pin in the cylinder to release it. The latch does NOT go into the hole and I've tried for an hour to guide it in there! It looks as if the thing is bent! I have no idea how it could have gotten bent. Also it is very dirty. grungy. nasty and filthy in there! Sheeesshhhhhhh...
Sometimes the hole isn't perfectly rounded and after repeatedly being forced, it gives. In any event, glad you found the problem. If you check on the S&W forums, somebody will have the latch you need as well as giving you advice on how to smooth out hole.
That operation might be above my pay grade. If it were a 22 I might try it but a 357 Mag I want to be sure everything is done properly. I'll get some pics tomorrow after a good scrubbing out. Would it be ok to use carb/choke cleaner to spray out the internals with?