Enter the movie title or phrase you want to search for, and choose which DVD rental company's site to search. The other search option is the Internet Movie Database site (IMDB). There are no brains built into this search - it simply calls the same search feature you get on that company's site.
To search at ZIP you must be logged into your ZIP account - the ZIP site no longer allows searching otherwise. However, ZIP enables you to sign up for a "browse-only" account fairly easily if you just want to look around.