Koala can be controlled by most modern midi devices (it needs to be USB class compliant ).
If on iOS, you need a USB adaptor for your iPhone or iPad – it must be the official Apple one, amazon/ebay knockoffs are unreliable.
For more info about plugging a USB interface into your device, see here
On Android, you may need to plug in your USB midi device before opening koala for it to register.
Koala responds to notes pressed on your keyboard – you can change the midi notes it responds to by pressing the menu button, then looking for the settings button. In the settings there is a MIDI tab with options for how the midi should work.