The Add button lets you add an application to the currently active list in the application preferences dialog box.
When you press add, you will be presented with a dialog box asking you to find the application that you want to add to the list. Navigate through your folders and files until you find the right application and select it to add it to the list.
Note: the Add button can also be used to update the name of an application when necessary. For more information, see the note at the end of the previous help topic.