First, please upgrade you Edge Animate to 2014.1.1 if you have not.
For general elements on stage:
You should find that option as the second button in the Color section of Properties Panel.
And for stage:
You should find that option in the Stage section of Properties Panel.
You might have to resize the properties panel to increase its width, as if you decrease its width then after some point that will hide some UI controls.
And if you still couldn't find that button, please share a screen snapshot of Edge animate.