Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
If you have snapping on, objects should snap to a guide you have placed at exactly the middle of the document.
 
Option-click the eye to make the text layer the only visible one, then click on its thumb to select the layer. Edit->Select All. Choose the Move tool from the palette, click the Align Vertical Centers button followed by Align Horizontal Centers.

(the hiding isn't strictly needed, but helps to make sure you're really selecting what you think you're selecting.)
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.