This is probably a very simple question that will make me embarrassed once I realize the answer.

How,in the simplest terms, do I use plugins in Gimp?

Well there's tons of plugins in The Gimp...Just look under "Filters" or play around with the options in the top menu bar of the Image screen.

Yes, but if you were to download plugins, how do you access them? The closest I've come is the plugin browser in Xtns menu, but it's just a list of the plugins. I can see the plugins I've downloaded there. But they aren't in the filter menu, so I'm lost as to how I can use them.

Well if you ever find out , let me know. I gave up on trying to instal them.

Oh well you would have to make sure the plugins are compatible first of all, and secondly all you would have to do is install the plugin into the C:\Program Files\GIMP-2.0\lib\gimp\2.0\plug-ins folder

It seems they are all executables, and if the plugin you have isn't an executable, you can try installing it in C:\Program Files\GIMP-2.0\share\gimp\2.0\scripts

I guess you have some experimenting to do :)