Actually, the only way to map entities in OGM is to list them in the persistence.xml, using the <class> tag. Since JPA provide the possibility to automatically detect which class to map simply scanning for @Entity annotated classes, it would be a great improvement to add this support in OGM. I've already tried to make it work, but it seems OGM can't make it. I even added the JPA facet to my project and enabled the autodetection, but it still didn't work.
Hibernate OGM 4.1.3, on MongoDB 2.4