As andrucz stated in his comment, the real cause of your problem seems to stem from a missing class.
Try checking whether you correctly imported your library ( and make sure that it contains the class EnglishUnknownWordModelTra‌​iner in edu.stanford.nlp.parser.lexparser.
(If you’re using Maven, verify that you correctly added the dependency – a quick google brougt this up : Stanford Parser Maven Repo )