Usually, 3rd party plugins come with an installer who puts 'm in the plug-ins folder, which is in the illustrator folder. Restart Illustrator before you try to use them.
I (also) meant: If there is no installer, you can (quit illustrator first and) put them in the plug-in folder yourself, and restart illustrator. This should be enough to use the plug-in(s), if they are suitable for your app.