If you want the popup be centered when you click on a link, insert the following code in the link inspector:
name: onclick value: $('#popup2').mb_open(); $('#popup2').mb_centerOnWindow(true) (replace the 2 behind the popup with your popup id !!)
If you want the popup be centered at open window, download the following snippet past it in (page info-header-javascript) and replace the 2 behind the popup with your popup id !!