I have just plugged in a Hitachi TV. I have connected the box to AC power, the TV screen to Ac power and the hitachi cable between box and Screen. The box has a green light, the TV screen has nothing. I have tried the ON button on the screen (under the TV), but still no action from the TV screen. Am I missing something incredibly easy?

Most likely it is 'something incredibly easy', but until you give us some details it will be hard to help you.

TV model number? How is it connected to the set-top box (cable type, connector names)?

Daniel