I have a combobox ( named "ddCity" ) placed on the first page of a
worksheet. On the Activate event of ANY other worksheet, I want to
display a polite message indicating they need to choose a city, first.

How do I reference the combobox?

Something like...

If isnull(Worksheets("Main").ComboBox("ddCity").value ) = True Then
MsgBox "Please Choose A City."
Worksheets("Main").ComboBox("ddCity").Select
Exit Sub
End If