i tried several usb keyboards. some work, and some others don't. try a usb hub. in my experiences, a usb hub with or without an external power enables you to use your usb keyboards that don't work when connected directly to the camera connection kit. it's only my guess: i think that the usb hub effectively manages amperage between the devices. additional power is not the solution, because most keyboards use 5 V, which is well covered by idevices.