You need to have the SAP R/3 integration component installed on the SAP server
IN the Client context, expand your list fo clients, double-click on the SAP server, it should give you a list of Installed components, and SAP R/3 needs to be there. If it is ther, in the Backup context, when you expand Backup PSecification, there should be a sub-category for SAP R/3
If you are not seeing the SAP R/3 component installed, right click on the servr, pick Add components, and follow the wizard