Create a Userform with a Label containing your message.
In the Visual Basic Editor, right click This Workbook and select View Code. Copy and paste in
Private Sub Workbook_Open()
Application.Wait (Now() + TimeValue("00:00:03"))
Save and close the workbook then reopen it to see your message.
"00:00:03" pauses for 3 seconds - change that and the name of the UserForm to suit.