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: https://www.koalasampler.com/help/audio-interfaces/can-i-use-koala-with-an-audio-interface/
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.