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 selecting the respective checkbox.
Please review the Managing Packages tutorial for more details.