You can't unless you only need to get the zip files from a single webpage.  For
example, point to the page, then use filters to exclude everything and then
only allow .zip

<http://www.server.com/folder/list.html>
-* +*.zip

If you want only zip files from a whole site, you still need to grab the pages
which link to them, otherwise HTTrack has no idea where the zips are!