The Add-on link when clicked will direct you to a list of add-ons that are connected to the database. You can add new add-ons or delete current add-ons. 

To delete an add-on, simply click the red trashcan and then confirm the deletion.  
To add an Add-on to the database, click the plus sign in the right hand corner. You can then select which add-on you would like to add to the database from the drop down list.  Once you have selected the new add-on click the blue save button. 
Remember, if the performance level of hosting is Economy, add-ons are not enabled for the selected database.  The database hosting must be upgraded before add-ons are available for the database.  Contact the system administrator, if upgrading is necessary. 
If you are adding and Add-on that needs to be deployed to the Act! for Web servers, you will need to manually deploy the Act! for Web Services again for the database. You can do this by going to the servers for the database, then deleting the Act! for Web Services then adding them back. It will take a few minutes to deploy the Act! for Web services again and all logged in users sessions will be terminated.  More info here on how to re deploy the Act For Web Services