Setting listbox items selected

In the "Selectionchange" of a worksheet a form is populated with items from the worksheet. One of the controls is a multiselect listbox that has several classes listed. I want to pre-select the classes that match the details of the worksheet. For example;

Worksheet Class1 through 6 has data 3, 8, 9, 11, 14, 16. The list box has items 1, 2, 3, 5, 8, 9, 11, 12, 13, 14, 16, 18, 23, 24, 32.

When the form is shown I want to be able to see the selected items in the list box where I will either select more or deselect some.

Thanks.

The data is located on the worksheet. Unfortunately (I just found by stepping through) the list box is not populated when I thought. Usually the list box is populated on a combobox change event. So I have to figure out how to fill the list box first, then select the items.