Why are the controllers not working in Super Meat Boy on OS X

Unlike other characters, holding the Shift button (or the Y button for those with a gamepad) does not maker her run faster. In her case, it activates her special -- the color change and slowing of descent you already realized.

What it sounds like you haven't realized, is that when you let go of Shift, you'll perform an air dash, but become locked out of performing another slowfall / airdash until you touch another surface.

Something like this is rather hard to explain with words alone, so I direct you to this YouTube demonstration to clarify.

It is possible to simply drop from the Key's platform back to the ground below, avoiding the walls entirely. It is also possible (and perhaps intended) to simply wall hop between the two walls(though if you haven't been playing the dark world levels, you may not have had as much practice with the precise timing).

Here's a video displaying a successful run of Omega -- and remember to not give up! Getting to the key is the hard part!