The HR24 uses the same codes as the HE23 & HR22. Inn fact, most of DirecTV's receivers use the same IR codes across the line. If it doesn't work, then most likely the problem is with either remote address pairing or IR/RF remote usage.
DirecTV receivers and remotes have an ability to set remote addresses. This is allows multipe receivers to exist in the same area, but only the remote and receivers which share the same address will respond. The Sling Media provided remotes are typically using address #1 since that's the default address for DirecTV. I would recommend going into the receiver's setup menu to make sure that it and the remote has an address set to 1.
The other issue is IR or RF compatibility. Another issue is whether or not the input is set to respond to either Radio Frequency (RF) or InfraRed (IR) commands. The Slingbox only has IR Blasters and cannot project RF signals. While you're looking in the setup menu options for your DirecTV receiver, you should also make sure it is set to IR instead of RF.