populate listbox depending on what's in another listbox

[Mondego]

i am using MS Access v6

i need to query a table, but only where a field in the table equals a field in a listbox.column(2)

I have 2 listboxes, the first has a list of parent categories, the second has a list of child categories. i need to click the parent cat listbox to populate it's children in the 2nd listbox, but am having issues with the Row Source for the child cat listbox.

i wrote this for the child listbox:

SELECT expense_categories.expense_cat_name, expense_categories.expense_cat_desc, expense_categories.expense_cat_id, expense_categories.expense_cat_parent_id
FROM expense_categories
WHERE expense_categories.expense_cat_parent_id=Me.ExpenseCatParent_list.Column(2);

and put it in "Row Source"

but a box keeps coming up saying "undefined function 'Me.ExpenseCatParent_list' in expression"

can anyone provide an idea on how i can achieve what i need?

thanks,
-mondego