...Sligs can't jump.
From what I can remember good jumping is one of the hardest things to achieve. I suppose it depends what kind of jumping you want. Just jumping up is easy but making it so that your character can move in mid-jump is very difficult.
I'll try and give you a quick way to achieve fixed jumping upward.
-Select your character Object
-Add Event = Keypress "Up"
-{action} Move Fixed : self ; direction=up ; speed=(strength of jump) ; relative=tick(I think)
-{action} Set Gravity : self ; direction=270 ; gravity=(strength/speed of falling) ; relative=no
-Add Event = Collision "floor Object"
-{action} Set Gravity : self ; direction=0 ; gravity=0 ; relative=no
-{action} Move Fixed : self ; direction=down ; speed=0 ; relative=no
I think that's it. Does it work?