First check the current source. Press the Input button on the TV (second from the left) or on the remote (below 9) until the correct Antenna input (Ant 1) is selected. Make sure that the antenna is connected to Ant 1 (the RF connection closest to the back HDMI ports). You only use Ant 2 if you have a coaxial (RF) cable connection. Then press Menu and then try the Setup menu again. You should be able to access the Installation menu and Terrestrial sub-menu now. Change the Input Configuration for Ant 1 to Antenna before attempting to Program Channels.
If this doesn't work, try resetting the TV to the factory settings. You'll have to access the Menu, go to the Installation sub-menu and then select the System Status and then System Information. Select Factory Defaults and enter the PIN. See the manual for the default PIN if you haven't changed it. Then confirm the reset. The manual is available from the product support page: http://support.toshiba.com/support/modelHome?freeText=2841536
. The direct link to the manual is http://cdgenp01.csd.toshiba.com/content/support/manuals/userguides/su2846615/42hl196_om_e.pdf
. See page 29 for the PIN. Then try accessing the Terrestrial menu and scanning for channels.
I hope this helps.