The menu item and toolbar button are shown by default, but they can be disabled if you choose to do so via the "Preferences" tab of the extension at about:addons. Did you accidentally toggle these options?

Another potential cause of not seeing anything is if you're using Private Browsing mode (or chosen "Never remember history" at the Privacy & Security section of about:preferences), without having allowed the extension to "Run in Private Windows". To allow access, check the checkbox after installation, or set "Run in Private Windows" to "Allow" in the "Details" tab at about:addons.