I assume you have a Slingbox Pro as this is the fourm which you posted under. Please correct me if I'm wrong.
Back of the Slingbox Pro
The Slingbox Pro only has an analog tuner. If the signal from your cable provider doesn't include any analog channels then the Slingbox Pro will not be able to find any channels. As far as I know due to the Digital Transition, As of June 12 2009, full-power television stations nationwide have been broadcasting exclusively in a digital format. This means an analog TV tuners will require a digital-to-analog converter box to watch digital programming.
Back of the Slingbox Pro-HD
The Slingbox Pro-HD has a hybrid analog/digital tuner. It is capable of tuning in over the air Advanced Television Systems Committe (ATSC) channels and unencrypted Quadrature amplitude modulation (clear QAM) channels. So, if your television provider broadcasts channels on compatible signals then the Slingbox Pro-HD will work for you.