That will depend on what is included in your cable subscription. If you're supposed to get more channels that what you're currently getting, then you will probably need an adapter box from your local cable company.
i usually just change the dial or press up or down.... but honestly I would subscribe for more channels thru your cable company. Orrr like I do stream it all from your PC to your PS3, xbox etc using Tversity or the like. Hulu sites stuff like that.
Try running the automatic channel detection. If you only had those channels before it programs them into the channel loop. So you might have more channels if you manually push the numbers while using the channel up/down will contain it to the presets. So yeah, try the auto channel function.