For you to have DirecTV service you must have a satellite set top box.
Your apartment complex might have their own internal on site receivers and are outputing them to modulators which in turn assign them to stations which are combined to create an internal cable network then redistributed to all of the apartment units. If that is the case you will find you have a very limited number of stations compared to what DirecTV offers, and the stations numbers won't match DirecTV's channel lineup.
At any rate, you can connect the incomming cable line from the wall to a 2 way splitter. One output goes straight to your TV while the other gets attached to the slingbox pro's ant/cable coaxial input.
Rerun the slingbox's setup wizard and yo should be good to go after it scans the stations to memory.