1. Go to your ACP
2. Click 'Style Manager'
3. Expand your templates
4. Find the header template, open it.
5. Find:
<td align="right" style="background-image: url($stylevar[imagesfolder]/menu_background.gif);" nowrap="nowrap">
6. Replace with this:
<br><td align="center" style="background-image: url($stylevar[imagesfolder]/menu_background.gif);" nowrap="nowrap">
Not tested, but should put your nav buttons on a new line and centre them.