OK, I am guessing that you are using what is called a separator on the back of your receiver. this separator is "Dish pro +" technology and is not compatable with a Legacy LNB (on the dish 500) you can do one of two things 1 obtain a Dish pro plus twin LNB and replace the one on your dish. 2. run a second cable from your dish to receiver and not use the separator.
I would suggest option 1 as it is the newest technology and will upgrade your system as well. Please reply if you need more help.