Hi,
Unfortunately you do not state where you are putting the code so I've assumed you coding a BeforeUpdate event on a form. With that in mind try out this code:
Dim Response As Integer
Dim strMessage As String
Dim strTitle As String
strMessage = "Type your message between these quotes"
strTitle = "Enter a title for your msgbox"
Response = MsgBox(strMessage, vbOKCancel, strTitle)
If Response = vbOK Then 'User chose OK
DoCmd.Quit
Else
Cancel = True
Response = acDataErrContinue
End If