Oh, I thought you'd be using one of our Picade products (https://shop.pimoroni.com/search?type=product&q=picade) I've not come across this encoder before, and it's not one we sell, is it an iPAC?
Unfortunately I don't have any experience with it, so there's not much help I can give short of educated guesses. It sounds like your buttons are shorting power to ground, though.
You might get lucky and find someone here who knows about your particular board, but otherwise you might be better off contacting the manufacturer.
...or using one of our boards ;)
(if we could keep the darned things in stock)
Edit: Apparently these forums are a veritable font of information: http://www.jammaplus.co.uk/forum/default.asp