AFAIK, this can only be done using some VBA programming.
First, you need to understand a little about events.
http://www.mvps.org/dmcritchie/excel/event.htm
http://www.cpearson.com/excel/events.htm
You would be interested in one of two WorkSheet Events:
Worksheet_Calculate :
This event occurs then a cell on the worksheet is calculated.
Worksheet_Change :
This event occurs when the value of a cell is changed. This event
does not occur when the value is changed as the result of a
calculation.
I'm assuming that the cell changes value when you manually enter a new
value. Say in cell A1. And the trigger value is 6.
(1) Right click on the WorkSheet tab.
(2) View Code.
(3) Place the following code snippet in the module.
(4) Change the value in Cell A1 to 6.
Private Sub Worksheet_Change(ByVal Target As Excel.Range)
If Target.Value = 6 And Target = Range("A1") Then
MsgBox "I've changed."
End If
End Sub
HTH
Paul