Films can be searched by genre within the catalogue simply using a subject heading search. While in the "Advanced Keyword" search option choose the term "Subject" from the drop down. Next name the genre you would like and follow it with the term "films." For example if you wanted to find all the horror films in the library you could select subject and type "horror films."
Don't forget to select "Film/Video or Material Type. Otherwise you will also retrieve books about that film genre.
Note not all genres (including "sub-genres") will have specialized subject headings. In this case there may be other ways of narrowing in on the results you want by identifying another unique characteristic of the kinds of films you want. For example, "anime films" is not recognized as a subject heading however performing the subject heading search "animated films Japan" for Film/Video materials reveals many anime films available through the Library.