You actually have a couple choices.. You could set the dish as 01: Round 18 and the switch type to multiswitch, this will get everything working (with the exception of HD channels and possibly local channels). You wont see any HD channels in the guide, but you'll get TV.. (Don't worry, this is temporary.. read on)..
Call DirecTV, and tell the CSR you need B-Band converters for your HR24, they can order them and have them sent to you (no charge). Once you get them, they connect between the receiver and the satellite in cables. Once thats done, repeat satellite setup (using the menu), and change the dish type to a 03:Slimline 3, then hit continue... once it completes, your all set.