Applications need to be included in a Package* in order to be available within the Add-ons tab.
To enable an application, go to your reseller Admin Area -> Left menu -> Manage accounts -> Packages. Click on the wrench icon in the "Actions" column of a plan. Select the application from the "Available Applications" list and add it to the "Included Applications" list by clicking on the arrows (on top) in between the two lists. Please review the Managing Packages tutorial for more details.