Alternative Centering Option
Another way to center your page is within the browser.
1. Create a large layer that is in the very back, and includes, all of the other layers in your PSD design.
2. Name that layer '_center2'
3. Convert. Everything will be centered in the browser!
Just one '_center2' layer is necessary, but all other layers need to be inside of that layer.
One major difference in output between '_center' and '_center2' is that the the left edge of your page will always be visible and will stay locked at the left edge of the browser. However, with '_center', the design will be clipped in order to preserve the 'centered' content.