Assuming its freeview and you know your local transmitter name, go to this site
http://www.unsatisfac...are.co.uk/dtt/dtt.cgi
Choose your transmitter and you will get the list relevant to you.
You will see that there are different MUXs
If you are adjusting your aerial, it is useful to know a channel for each Mux - digital signals are more fussy than analogue - so if your aerial is not spot on you may not get all muxs or signal breakup.
Also, again assuming its Freeview, if you only have it connecte via the aerial the it may not actually output a signal via aerial.
You should have it wired as
aerial in to the box and then the box to tv/video via scart or hdmi cable