Unfortunately, I'm not familiar with the menu setup for units outside of the U.S., as there can be differences in the options. You might check to make sure you don't have "cable" selected where "air" should be selected, and if that's not the case, you might want to consider calling the Samsung Service Center in your region to see if they can assist troubleshooting your unit.
We have just purchased a samsung series 5 lcd tv (LE37B551A). Even though it has built in freeview and we have a digital aerial we cannot pick up any digital channels. There is no DTV option on the source menu either. Please can you help us with this as it is very frustrating!