Owner

If you are displaying a message box inside Excel, then Excel is the owner by default.
For this reason you do not need to use the owner parameter inside a VSTO add-in.
Assigning the owner parameter has nothing to do with displaying in the Centre of Parent.

public static Microsoft.Office.Interop.Excel.Application gEXCELAPPLICATION; 

public class Win32Window : System.Windows.Forms.IWin32Window 
{ 
   private IntPtr _handle; 
   public Win32Window(System.IntPtr oHandle) 
   { 
      this._handle = oHandle; 
   } 
   public Win32Window(int oHandle) 
   { 
      this._handle = new System.IntPtr(oHandle); 
   } 
   System.IntPtr System.Windows.Forms.IWin32Window.Handle 
   { 
      get { return this._handle; } 
   } 
} 

System.Windows.Forms.IWin32Window owner; 
owner = new Win32Window(Constants.gEXCELAPPLICATION.Hwnd); 

System.Windows.Forms.MessageBox.Show(owner, 
    sText, sCaption, 
    System.Windows.Forms.MessageBoxButtons.OK, 
    System.Windows.Forms.MessageBoxIcon.Exclamation);