Open the Align panel; from the Window menu select Align. In the lower right corner of the panel there should be a control for Align To (if not, choose Show Options from the flyout menu in the upper right corner). From the Align To control choose Artboard. Any object you now select and choose to Horizontally or Vertically Align Center will center on the artboard.
Simply choose Align to Artboard in the Align Panel or Control Bar. Select the "Align To" button and choose "Align to Artboard." After that, any objects you select and use "Align to Center" will be aligned with the center of the currently active artboard.
