On the addon main page, under "About this extension":

"The ATH UI can be accessed 3 ways: In the sidebar, from the toolbar button popup, or in the full view window. Each UI provides 4 different views: .... etc etc etc"

(I don't provide instructions on how to use basic Firefox function, such as using the sidebar or using the toolbar buttons as I expect users to already know how to use Firefox.)

...along with the 19 screenshots which give descriptive images and detailed text for the use of each function.

EDIT:

For all addons that use a toolbar drop-down menu Firefox places a button in the toolbar, appearing with the addon's icon. Look for the button with the ATH icon (the big blue "A".)

For all addons that use the sidebar Firefox places the addon's listing in the sidebar menu (View > Sidebar.)

Using ATH in the sidebar allows the menu to be viewed at all times, whereas using the toolbar button drop-down is ad hoc.