Hi there Richard,
If you have just a message box ...
Dim Msg as vbmsgboxresult
Msg = Msgbox("Enter value:")
If Msg <> "" then Range("A1").value = Msg
.. if you have a userform, use the event/objects code ...
Range("A1").value = Me.TextBox1.Value
Replace the range/object for your specific situation.
HTH
Regards,
Zack Barresse