The battery for that unit should be located in the front compartment. If you don't see a battery, look for the battery cables that have been disconnected and are probably taped up.
A battery is required for the trailer to operate the emergency brakes if it becomes disconnected during towing. The break-away switch located on the pin box actuates the brakes if the pin is pulled.
You should add a battery even if the trailer will be only used on shore power - the battery helps "filter" the output of the converter.
Be careful when you are connecting the cables to the battery that you get the polarity right. The cable that goes to ground (usually a white cable) goes to the negative post.