Again, what version of RSLogix 500 are you running, and what menu are you having problems with ?
If you select Comms -> System Comms and then navigate down through the RSLinx drivers that you *do* have running all the way to the SLC-5/04 CPU that you want to go online with, RSLogix 500 should be able to go online if you click the "Online" button. Otherwise, an error message should appear.
The driver name makes me suspicious; "AB_KT-4" implies that four 1784-PCMK or 1784-KTX drivers have been configured at one time on this PC without deleting the others; either you have a bunch of DH+ cards or there's something wrong with the basic driver setup in RSLinx.
Are you sure you can browse down to this SLC-5/04 using RSLinx and the "AB_KT-4" driver ?