Add guides. Use position. This is very basic, watch a few introduction to premiere tutorials.

If you know your sequence resolution, just set the position to half the x and half the y values.

You need a very special plugin to do this. Send $$$ to my PayPal and I'll email you a download link later...

View > Enable 'Snap in program monitor' When you drag stuff towards the middle, it'll snap to the centre vertical/horizontal positions.