If you have a plugin that is not an RBZ, you can place it in the correct folder on your computer (see the link below). If your plugin is listed on the Extension Warehouse, I would just download from there (and EW will tell you if the plugin is compatible with your version of SketchUp).
Check out this post by ThomThom - he does a good job of explaining about the new plugin folder.
Let me know if this helps or if you still need assistance!