Going by your description, it looks like you've using version 2.1.5 or earlier of TicketMonster. In this version, most of the JPA model classes like MediaItem, are annotated as @Portable (an Errai annotation). To have this annotation (and the JAR that contains it) in your classpath, you'll need to follow the steps outlined in the first section of this chapter.
Note that in version 2.1.6 of TicketMonster, Errai and GWT are no longer used. The site would be updated later to reflect this. In the interim, you could use this release tag: https://github.com/jboss-developer/ticket-monster/releases/tag/2.1.5.Final to obtain copies of the older sources. Apologies about the confusion.