Oh, do you want to know how to jump, and then fall back down?
That's simple, here's what ya do: Go to keydown (choose the key you want/make sure you set the reapeat to disable) -> add action, script editor:
yvelocity = yvelocity - 8;
We now have the ability to jump.
Then add an event -> draw actor ->script editor:
yvelocity = yvelocity + .8;
We now have gravity to pull us back down.
All that is left to do is add ground. Make another actor called ground. Give it an animation, maybe just a line made from ms paint. When you are done, go to the "jumping" actor. Add an event -> collision -> physical response, On: any side, Of Actor: ground, Repeat this event while actor is colliding: yes -> edit action, leave everything as is EXCEPT the first number under Final Velocity Multiplyer. Change that number to zero. Click OK, immediate action. Go to game-mode, and try it out.
I hope I answered your question.