Hi Tim,
As Dylan says, there can be one or more access points for a plugin (aka. script, ruby.) A script can add a menu-item to any of the available menus, it can add a sub-menu with multiple child Menu items, it can add a Toolbar accessible under the View > Toolbars menu, it can use a context (right-click) menu, and finally it can make itself available in the Windows > Preferences > Extensions dialog.
Once you get the .rb file into the Plugins folder, the easiest way to load it is to close and restart SketchUp. If you don't want to close SketchUp, you can load the file by opening the Window > Ruby Console, and entering:
load "script_name.rb"
(but replacing "script_name.rb" with the filename of the .rb file you've installed.) This is also a good way to do it if you are having trouble with a script because you can see any error messages that would prevent the plugin from being loaded.