custom message box

Question

hello .. i download asp.net msg box from some where...

its working properly n display the msgbox at the center of page...

bt when i use master page it nt displays at center of the page. what changs i have to make in the mster file , or page or in stylesheet?

here is my code:

http://www.mediafire.com/file/31ym4qztywl/msgbox.rar