"Unfortunately the MS20 doesn't include an external PSU socket so you'll need to use a powered USB hub as a bridge between the two. I'm not entirely sure why but it seems not all powered hubs are equal to the iPad (or the MS20) and it wasn't until I tried my third hub (a small Belkin unit) that the iPad recognised the controller. Actually there's a free App called MIDI Monitor that can be useful in these situations."
more info: http://chriscarterchemistrylessons.blogspot.com/2011/02/ipad-ims20-controlled-by-legacy.html
Korg iMS20 controlled by Korg Legacy MS20 hardware from Chris Carter on Vimeo.