The easy way to center the menu is to choose ‘Top edge-Horizontally Centered’ at the ‘Position relative to’ option box.

The next step is to put the value you want at the ‘Horizontal distance from Edge’ box.

However if your site is designed as % and not specific size, the menu might be shown off place.

To avoid that you need to apply the ‘Position relative to a page element’ trick using an empty table with 1 row divided in 2 columns.

Of course this empty table will be placed where the menu want it to be. Its size also it doesn’t matter.

The <span> code should be associated with the right cell.
If you leave it as is, the top-left corner of the menu with match the top-left corner of the right cell.

To correct that, enter at Tools-> Compile Properties -> Advanced Scripting, the variable awmRelativeCorner=4 (as shown at help > Position the menu relative to a page element)