Today we’re launching add-ons—new tools created by developer partners that give you even more features in your documents and spreadsheets. To browse through add-ons for Docs and Sheets, select Get add-ons in the Add-ons menu of any open document or spreadsheet. Once you install an add-on it will become available across all of your documents or spreadsheets and you can start using it right away. Note that add-ons for spreadsheets are only available in the new Google Sheets.
Google Apps admins must enable add-ons through Drive settings in the Admin console. Users will always see the Add-ons menu and can browse them but will not be able to install them if the feature isn’t enabled by the admin.
Google Apps for Business, Education and Government
Reference by : http://www.google.com/