Nabi has their own proprietary usb plug for many of their tablets. I ordered a Nabi big tab 20″ for a display at work, but I needed to connect some usb peripherals to it. I assumed it had a standard usb micro port so I didn’t order an out of stock Nabi USB OTG adapter. Doah… USB OTG is really simple. There is a 5th pin, and when it is tied to ground, the device knows it should be a USB host instead of client. (aka, when that pin is tied to ground, you can plug usb things into it, instead of plugging it into another computer).
Here is my instructable if you need one of these yourself.