After much experimenting and frustration, I finally figured out how to do this. To manually map services to channels you must first create one or more empty channels to which you will map the services. This is backwards from what I would have expected. I would have expected the act of going from a service to a channel would create the channel.
Anyway, here is what you need to do to manually map services to channels.
# Select Configuration | Channel/EPG | Channels.
# Click Add Channel which will create an empty channel entry.
# Under the Number column assign a number. As far as I can tell you can pick any number, this does not matter.
# Under the Name column enter a name that is meaningful to you. This could be "ABC" or "KEYT" or "Fox channel 24" - whatever is meaningful to you.
# Repeat steps 2 - 4 for each channel in your area.
# Very important, click Save Changes else all the work you have done will be lost when you leave the page.
Now that you have the empty channels, you can map the services to them.
# Select Configuration | DVB Inputs | TV Adapters | select your adapter from the combo box | Services.
# For each service in the list, double-click the Channel name column (where it currently says Unmapped) and a combo box will appear. From the combo select the destination channel you created above.
# Again, click Save Changes else all the work you have done will be lost when you leave the page.
Of course, there is the question of how do I know what channel is what since the only info present is the frequency? Sadly the easiest might be trial and error. Just pick one and then go view the channel. Once you figured out what channel it really is, change the mapping to the correct channel. The other option is to lookup broadcast channels in your area, find the real (not virtual) broadcast channel, then correlate that real broadcast channel to the frequency. That is why I say trial and error might be easier.
One final tidbit that helped me. Once you have mapped channels, an easy way to view them is to use VLC (available for free on all major computer platforms). Run VLC then from the menu at the top select Media | Open Network Stream. In the URL box enter "http://<name>:9981/playlist/channels" (where <name> is the name or IP address of your tvheadend server) and click Play.
I hope this helps someone else having difficulty figuring out how to map channels in tvheadend.
