To use the MessageBox via client you should call the
ShowMessageBox(type,Header,Text,Width,Height) function.

The first argument is the type. At the moment possible values are info,error,success and warning.
The second is the the header of the messagebox and the third is the text. You can also use html...!
The other two are width and height of the message box.

Example : ShowMessageBox('info','A Simple Header','Just some text',400,125);