Each TV transmitter broadcasts on different frequencies. For this reason your box will need to be tuned to your local frequency.

Connect up the box as usual and make sure the power light is green.
Press "Menu" on the remote control and use the down arrow to highlight "Installation".
Then select "First Time Installation" and hit OK.

There will be a pause while the box detects the transmitters carrier signal.

On the next two questions just press OK.
The box will now search for the available channels. This takes a couple of minutes.
Once it has finished your freeview box is ready to use.

I recommend you do this when you first install a box to ensure you receive all available channels.