Hi IG,
I have one of these and it works well.
Have you checked/rechecked the basics.. that the red battery lead is attached to the pos terminal on the battery and to the reddish screw fitting on the back of the unit. Same visa vi with the black lead.
If connected ok, then there are two lights on the front panel next to the switch, green and red.
When you switch it on, with a load connected like a 240v light, ... do either of the red or green lights illuminate??