In order to get all available channels, I went into MythTV Backend Setup -> 3. Video Sources, and after selecting my source, which is schedulesdirect.org in my case, I changed the Channel frequency table from 'default' to 'us-cable.' Now I can view all my cable stations.
Source: http://ubuntuforums.org/archive/index.php/t-632896.html
Source: http://ubuntuforums.org/archive/index.php/t-632896.html