How an extension is activated and used depends on the extension, and the type of tool it contains.
Look at the extension’s documentation for information about the specific extension you are working with.
Here’s an overview of the possibilities:
Extensions which add an entry to the Extensions menu
This is the standard mechanism for making extensions available, and by far the most common case. The extension adds an entry to the Extensions menu, with or without a submenu. From there, one or more dialogs can be opened.
Extensions which add a tool to the Tools menu
Some extensions contain interactive tools which are used when editing glyphs. These extensions make themselves available by adding an entry to the toolbar at the top of the Glyph Editor.
Extensions which add new entries to existing application menus
Some extensions add entries to other Application Menu items instead of Extensions. This makes sense for extensions which add functionality to objects which are already present in the main application menu, such as Font, Glyph or Window.
Extensions which add a new entry to the main application menu
While not very common, this is a useful solution for extensions which require lots of menu space. Instead of cluttering the Extensions menu, the extension can add its own menu to the main application menu.
Extensions which do not add any visible UI control
Finally, some extensions don’t add any menu item or tool. They simply do their job silently all the time, without drawing attention to themselves.