How do you enable the layer based collision matrix?

I’m trying to get at the layer-based collision matrix described here:
http://unity3d.com/support/documentation/Components/Layer%20Based%20Collision%20detection.html

However, my Edit->Project Settings->Physics menu is missing the collision matrix at the bottom of that one. The about screen shows that I’m running version 3.5.0f5.

Anyone know why I can’t see that part of the menu?

Thanks!

I found the answer while the question was in moderation - asafsitner nailed it. My inspector was in debug mode. I switched it over to normal mode and the collision matrix showed up. Thanks all!