You can use the "align & distibute" coomands from the draw toolbar

First choose "relative to slide" and then align > middle followed by align

You can record a macro if you do this a lot

Jeff,
You could use align and distribute, you can find it on the drawing toolbar,
click on draw, choose align and distribute, drag the menu away with the rear
off ribbon on top. Click on the last button on the toolbar, which lets you
align an object in respect to the slide. Choose center and align center
(second and fifth button from the left).
A second possibility, activate the guidelines, View - grid en guides - check
guidelines and snap to grid, now drag the object to the middle of the slide.
If you use snap to grid it is very easy to center the object.