To do this in the past, I've created a dummy beam with very small size (like 1"x1" cross section) running along the top of the wall and just applied the load to the dummy beam. Note that the load will be distributed to the wall as point loads at mesh or node points though. So you'll want your wall mesh fine enough that the loads transferred actually somewhat resemble a line load.
Thanks again for your reply. So you cannot really put line load on top of the wall, unless you put a negligible size of beam on top{that's the only way isnt it?)
To my knowledge, no. I want to say you need the dummy member too. I think I tried just drawing a null line and applying the line load to the null line, but the program would not allow me to do this. I might be mistaken, though.