Please Note: GAFE administrators can disable add-ons through the DRIVE Settings in the Admin Console.
Once you install the add-on, it will be available across all your documents or spreadsheets.
April 1, 2014: There is a new add-on feature in Docs and Sheets. You can see the add-on featurein the menu of an open document. Click on the link in the drop down menu to browse through choices.