You should get 120vac between each straight contact and the D-shaped (ground) hole. That test device may not be able to test a 240vac circuit. A multimeter connected between the two straight slots should read 240vac, and from either one of them to the D-shaped (ground) connection, it should read 120vac. Not reading 120vac from both slots to the ground means either the receptacle is not wired properly, or there's a problem in the wiring, or the breaker is bad.
If you do get 120vac from both slots to ground, and it still doesn't work, then the unit you're plugging in is bad unless you've got a really weird situation where the same lead is connected to both of the slotted connections, and there's no difference between the two, but you can still see 120vac.