How to configure hibernate subclass extends attribute?

I have a hibernate mapping config that has a subclass defined with an extends attribute set. but I'm getting the error

org.hibernate.MappingException: Following superclasses referenced in extends not found: ...

I've added the jar file containing to my classpath but I still get this error. So I have no idea where it is looking for this class. What am I supposed to configure to point to the jar so the class can be found?