First, make sure you're using the latest MC20. Earlier versions of MC20 didn't support skins directly; you had to do some hacks.
Second, yes, you put them into the Application Support/Media Center 20/Skins/Standard View/ folder. The skins you downloaded will have a mirrored directory structure that has a skins/Standard View/ folder inside. You may also have custom resources, images, web pages and that kind of thing. They *can* go in the Data directory (in subdirectories as named), but they are optional. It's weird because those Resource things are global: They apply to all skins. So you can't really switch them on the fly.
Skins are picked up while MC is running. As soon as you install them and go to the View > Skin menu, you should see them; no restart required.
Good luck.
Brian.