The action is "Apply Impulse at Angle" and the Parameter is the formula to find out the angle between the player and the object to apply impulse to.
So whenever you select an action (almost any action) there is a window with a textbox that you can type into to input X and Y or whatever you are inputting. Above (I think) that is a greyed out textbox that appears, and becomes opaque when you hover over it (@Ashley I think this is a bad UX decision). Once you've found that, you can pcik the System object, then scroll down and pick the angle expression, and it will help you along.
Once you get more used to this sort of thing it will become MUCH friendlier to say sytem.angle(x,x,y,y) or object.Platform.VectorY than it will be to say click this choose that do this. =) Good luck!
Play around with it, if you fail, put together a capx and link to it here and we can look into it.