Problem solved, folks! It turns out that there was a small icon in the upper right-hand corner of the screen I hadn't noticed before, next to the icon for the update manager. I clicked on it, and a menu dropped down that had all the information I needed to enable my mobile broadband to work with mythbuntu just fine! Unfortunately, my system got messed up while I was downloading and installing a number of updates, so now I have to reinstall the whole system! Oh, well, two steps forward and one step back . . .