Assuming you just want 2 buttons (text,sprite in this example), 1 for zooming out and 1 for zooming in instantly and everything in 1 layer.
<img src="http://i42.tinypic.com/jkivi9.png" border="0">
Heres an example assuming that you want the zooming in and out gradually, have a HUD layer with two buttons (text,sprite in this example)
<img src="http://i42.tinypic.com/2vahz0o.png" border="0">
I'm not too sure of how the lerp works, I just edited the driving example in Construct 2 to zoom in and out. But here's more about lerps.