Set the property "Use default controls" to "No" in the platform behavior properties.
Add a keyboard plugin, and use the events "on key pressed" or "on key is down".
It's explained/showed in the pages 4 and 5 of the tutorial I previously linked.
When you use those condition, you use then the action "Simulate control" accordingly to the key pressed. (ex: left key is down => Simulate control Left)
You then use the "simulate control: Jump" for the key of your choice.