Well, it's not obvious, but it is pretty simple ...
1) Select the element with the Arrow Tool
2) If it's not already visible, go to the Window>Show Control Palette menu command to open the Control Palette
3) Take note of the page dimension, then halve it -- e.g. 8.5" for a letter/legal page in portrait orientation = 4.25"
4) Click on the center dot in the nine-dot proxy at the left side of the Control Palette
5) enter the center measurement (4.25" in our example above) into the "X" edit box on the Control Palette, then hit the enter key
This should mathematically center your element on the page. You may determine that some adjustment is necessary to make a lopsided element seem optically correct.
Good luck,
Randy Hagan-htech
P.O. Box 15055
Portland, ME 04112-5055
phone: 207-332-5822
email: help@RandyHagan.com