The plugin developer's guide is available from the Resources section of the AGS website.
Plugins come as DLL files with the names AGS*.DLL, for example agscircle.dll might be a plugin providing a DrawCircle text script function. To use a plugin, place it in the AGSEDIT directory (not your game folder).
Start the AGS Editor up with the plugin in its folder, and load your game. Go to the Game menu, and select Plugin Manager. There, you should see all available plugins listed. To use one in your game, check the box next to it and press OK. The plugin developer should provide instructions on how then to use their plugin. Save your game to make sure that AGS remembers that you want to use the plugin.