Head over to The Access Web--Dev Ashish runs the site, and it's pretty doggone complete. You need two things: how to add "all" to a combo (go to search page, enter 'combo' and it should be one of the first items), then in the query you're using to return recs, I'm assuming you have something along the lines of
forms![formname].[comboname] in the criteria line associated with the combo. Put in
forms![formname].[comboname] or forms![formname].[comboname] is null
this is also on his site, if you search using the work Parameter.
HTH
Ben