I should elaborate by saying that you'll need to put this line of code in the
Worksheet_Change sub of the sheet that holds the condition. For example, if
you wanted a message to appear after cell A1 on Sheet1 reaches the value 100,
you'd
want something like this for the Sheet1 Excel Object:
Private Sub Worksheet_Change(ByVal Target As Range)
If Range("A1").Value >= 100 Then _
MsgBox "A1 has reached limit", vbOKOnly, "Alert"
End Sub