Pop up message only on FIRST opening, and not to display again

[dsrt16]

I have a pop-up message that Mac users need to see the first time they open the file, but it would be annoying to have that pop-up every time they open. How do I tell it to only run this code the first time they open the workbook?

Private Sub Workbook_Open()

#If Mac Then

 MsgBox "This program detected you are on a Mac computer. While I changed the code to mac-friendly code, some Macs still do not read the code to create your drop-down menus. If your drop-down menu items go on one line instead of each item on a separate line, then read the troubleshooting for Mac file."


#Else

  'Do nothing

#End If


End Sub