Are you writing about the button or the combobox from the control toolbox
toolbar?
If it's the button, I don't understand.
If it's the combobox, go into design mode (an icon on that control toolbox
toolbar).
Right click on the combobox, select properties.
Look for ListFillRange
and type in the address that will contain the list.
(like Sheet2!a1:a10)
And you may want the LinkedCell (also on the properties dialog) to be the same
as the cell "holding" the combobox. (I'd give that cell a custom format of
";;;" (w/o the quotes). It'll look empty when looking at the worksheet--but you
can still select the cell and see the value in the formula bar.
And change the Style property to "2 - fmStyleDropDownList"