Most plug-ins that are required for your day to day use of Maya are automatically loaded. If you get an error message in the Help line when you try to access a menu item or tool, or if you discover that an expected menu item is missing, this can mean that the feature plug-in is not loaded.
If you suspect there is a plug-in not loaded in your scene, check to make sure the plug-in is loaded using the Plug-in Manager (Window > Settings/Preferences > Plug-in Manager). See also Load or unload Maya plug-ins.
For more information on features controlled by plug-ins, see List of plug-ins included with Maya.