Rightclick the sheet tab in question, find the selection-change event and
call it from there after validating that it's the right cell, like

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target(1).Address = "$B$12" Then Call MyMsgboxMacro
End Sub

If I were a catholic I'd go to church tomorrow and confess "Father, I have
sinned, I helped a stranger on the internet to create this terrible thing".
A MSGBOX when you enter a cell ? How can that be anything but annoying, what
good can it possible do, how would you react to such a thing, would you
read, digest and obey ? Don't, please don't.

Best wishes Harald

"James Hamilton" skrev i melding
...
I want to write a macro which brings up a msg box when a particular cell
is
selected.

I've written the macro, and when I run it from the VB editor the message
box
appears, but when I go back to excel and select the particular cell, it
doesn't work.

How do I "assign" the macro to the cell?

Thanks.