How do I add data into combo box using vba?

Question

I want to add unique data (From MainMenuData worksheet in same workbook) into ActiveX combo box placed on sheet1. Changed ComboBox Name from Properties to ZoneDropDown. Wrote following code on workbook open event.

Private Sub workbook_open()

Dim Zn()

Dim R As Integer

For R = 1 To 248

    ZoneDropDown.AddItem Sheets("MainMenuData").Cells(R, 7)

Next

End Sub

It gives following error for ZoneDropDown

Microsoft Visual Basic for Applications
Compile error:

Variable not defined

But if I place button on same worksheet (i.e. sheet1) above code execute successfully and data added into combo box.