Sounds like you may have a superdish (arm with 3 lumps). It should have a box like set at the end and then a single one off to the side. If you have a superdish you will have a DP34 switch somewhere in the system (3 lines from the dish to the 34 switch and "1" for each receiver coming off of it).
That being said, you can't "split" one of the existing lines, each line needs to be a "home run" from the dish (there may be a ground block somewhere but don't worry about it). Run a line from the "bottom" side of the 34 to your receiver, run a switch test (menu, 6, 1, 1), Activate it, and you should be good to go.
WTT