You have learned to align shapes in PowerPoint 2011 — for alignment to work at all you need to have multiple shapes or other slide objects selected so that they can align to each other. However, you may want to align just one shape (or even a group of shapes) to the exact center of the slide. Fortunately, that is as easy to achieve as well.
Learn how align a shape to the center of a slide in PowerPoint 2011 for Mac.

