In the new \"Xcode 6\" the ability to change the center of measure of a button is missing.
In Xcode 5: http://imgur.com/jWHJp4v
Xcode 6: http://imgur.com/rsNayVZ
If your layout isn't too complex you can "Pin" your button or view to the top or bottom, sides, etc.
Or you can do your constraints manually here:
The first method works really well and is super easy! I had a similar problem as you looking for tools that have been left out in Xcode 6 and found this wonderful tutorial on Auto Layout:
http://www.raywenderlich.com/50317/beginning-auto-layout-tutorial-in-ios-7-part-1
If you are trying to center it I would click on the button or view you are working on and look at the size inspector here:
Look at where I circled and make sure that the bottom space and the top space are equal to the same amount (mine aren't equal), but that should give you the center.