Short answer - you can't. There is no way for you to manually add a channel or scan for them like the plain H20 HD unit can.
Long answer - D* pulls the channel info from a database and sends guide information for what is supposed to be available in your area. It is up to your local station to report what sub channels they have to the company that maintains the database (can't remember their name), they then provide that data to D*. It appears they have not reported the sub channels to the company, therefore it is not included in the guide data that D* has.
I had a problem with my FOX station. It showed up in the guide but would not pull in a signal. At first I thought it was bad guide data but turned out the station did not have their primary/secondary data set correctly in their feed. So even though I had guide data, the HR20 would not display the channel since the data did not match. FOX locally corrected the issue and the channel came in.
You will need to probably contact your local station and ask them to update their channel information with the company that maintains it (sorry, cannot remember the name). Once done, you can try to rerun the OTA setup and see if the sub channels show up.
My NBC station has 3 subs with -4 being a weather channel. It does not show up in the guide but all my other station's subfeeds do. I'm not worried about weather so I have not bothered to call them. I assume they will update it one day.
