Any class compliant MIDI controller can connect to an iPad or iPhone using the Apple Lightning to USB 3 Camera Adapter. Get a CoreMIDI-compatible app such as Garage Band, Tabletop, NanoStudio, MusicStudio, or Thumbjam. Connect the USB cable to your key controller and the Lightning adapter. Plug the adapter into the iPad making sure that the app is launched before the connection. Tap some keys to check that power is getting to the instrument.