Some plugins also need to be turned on, or the toolbars to be shown.
First (on PC), go to the Window Menu and open "Preferences" and look for the "extensions" tab. Scroll through the extensions and make sure they are all turned on. Many plugins do not use this extension manager, so do not be surprised if your plugins are not listed here.
Then go to the "View" menu and go to "Toolbars" and make sure that any toolbars for your plugins are turned on.
Oh also, you need to restart sketchup after installing plugins.
If neither of those help, then definitely post images of your plugins folder.