It does seem rather strange that the manual that came with this inverter doesn't tell you how to use it?

 I'm assuming your battery charger is 240 v a/c,, and usually those are quite hefty units, can this inverter handle the load?

 Typically, (in the united states) you will have 120v on one and 120 v on the other hot,,so between these two you get 240 v. between the 120 v line (either one) and ground you get 120 v..