The Gmail Addon must be enabled from the Admin Console. To do this, log in to Gmail as the Administrator, and click on the cog to the top-right, as per the screenshot below. Then, click on 'Manage this domain'
Doing so will take you to the Admin Console.
Navigate to the following page:
Apps -> Marketplaces Apps
This will take you to a page displaying your currently installed add-ons. From here, click on the + button in the top-right, as highlighted in the screenshot below. Then, search for 'Actionstep' and click on the Actionstep add-on when it displays. You will have the option of applying either 'Domain Install' or 'Individual Install'.
Selecting 'Domain Install' will allow you, as the admin, to install the add-on for all users within your system. You will be able to enable the add-on for individual users from within the admin console.
Selecting 'Individual Install' will allow each individual user within your system to find the add-on from the Google Marketplace and install it within their Google Apps console.