Missing HDTV Signals

I set up the HD Over The Air channels on my HP M8300F and receive the main
channels. Can't see the second, third or fourth channels on each main. I
get 10.1, 19.1, 25.1, 35.1. How do I find 10.2, 19.2, 19.3, 25.2, 35.2,35.3
etc?

I'm only using the Media Center for OTA HDTV not cable or satellite.

After I find them, how do I get them to show up in the guide?

Thanks.