If it's just the one combo on the main form, you can do that just by setting
the Link Master Fields property of your subform to the name of the combo,
and the Link Child Fields property to the matching subform field. No code
needed. (If nothing is selected in the combo, the subform will show
nothing.)

If you want to be able to filter your form by different fields, this might
help:
   Find as you type - Filter forms with each keystroke
at:
   http://allenbrowne.com/AppFindAsUType.html
The idea is that you choose a field to filter by (combo), and then enter a
value to match (text box), and it filters the form as you type. You don't
need to make any changes to the code to get it to work with your form: just
copy'n'paste, and set one property.