You would need variables to hold the starting point of the jump. Then at the end of the jump you can compare the points from the saved starting point with the point that the player is at to figure out the distance.
You'd start by making two global variables:
In the event that starts the jump (let's say pressing the spacebar) you would do something like this:
Keyboard: on key "spacebar" pressed
- System: Set StartX to Player.X
- System: Set StartY to Player.Y
- (do your jump code here)
Then, once the player lands you would use:
distance(StartX,StartY,Player.X,Player.Y) to find the distance.