' On application window
Private Sub UserForm_Initialize() 
   Me.Top = Application.Top + (Application.UsableHeight / 2) - (Me.Height / 2) 
   Me.Left = Application.Left + (Application.UsableWidth / 2) - (Me.Width / 2) 
End Sub 
' Or
With myForm
    .StartUpPosition = 0    ' Manual
    .StartUpPosition = 1    ' CenterOwner    
    .StartUpPosition = 2    ' CenterScreen
    .StartUpPosition = 3    ' WindowsDefault, screen top left
End With