For CS4/5, CRL+Home centers a layer in the Comp Panel. For earlier versions of AE, select a layer and drag it towards the center. As you drag, add CTRL followed by SHIFT (both modifier keys held down) and your layer will snap to the center of the Comp Panel.
To align layers, it's pretty straight forward as the icons are self-explanatory. I'm taking a guess that your query is actually about distributing layers.
For example, to distribute layers horizontally across a range of pixels; start off by placing one layer at each end. Ensure that the boundaries of the other layers do not extend beyond these two layers. Think of these two layers as edge markers for your distribution area.
Now select all the layers and hit the distribute horizontal icon. If you set up your layers as prescribed, it doesn't matter which distribute horizontally icon you click.
AE Training in South East Asia. AE templates for sale and rental. Click here for more
I have Ctrl and Home keys but they don't seem to center the layers. Sometimes shortcuts don't work on laptops because the keyboards are abridged, and the keys are sometimes combined with other keys, limited, etc. For some reason, those shortcuts didn't work. Let me know if you find shortcuts for the MacBook Pro that work. Thanks.
Generally speaking, any shortcut that uses Control on a PC will use Command (the Apple key) on a Mac.
On a Mac, the shortcut to center a layer is Command-Home. On a MacBook Pro, it's Command-Fn-LeftArrow.
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting RenderBreak Blog - What I'm thinking when my workstation's thinking
Creative Cow Forum Host: Live & Stage Events