You can set the Attributed String of the menu item, which does allow you to center the text, as well as change the font, color and size of the text as well.
I do not know which version of Xcode/OSX you are using, but under Xcode 5.0.2 you can use the built-in interface builder to edit the attributed strings of menu items.
Click the MainMenu.xib entry, then click the menu you want to edit to open it, then click the menu entry. On the right side of the window there should be some options, if not click the right most square in the group of 3 at the top right of the Xcode window 1.
Once you have the right panel open click the Attributes Inspector icon [2] (4th from the left, looks like a slider type icon). At the bottom of the list of attributes you will see an entry for "Attrib. Title" [3].
That is where you can set a custom font, color and alignment for your menu item.
If you need more control then that provides, you can also embed custom NSViews into your menus which gives you complete control over how the menu will look.
