It will work with Platform behaviour, using the Set Vector Y event. For checking the vertical speed you'll need to use an expression like Player.Platform.VectorY, as there isn't a condition for checking the individual X and Y speeds.
So like this:
<img src="http://img7.imageshack.us/img7/3298/screencap01.png" border="0" />
Just swap out Space with whatever you want as input (in your case, probably the mouse), swap Player with whatever object you have that needs the variable jumping, and tune the numbers a little to see what looks best.
Hope this helps you.