You probably just have the Java EE 5 jar in your classpath, but that's unfortunatly not enough, because this jar lacks some method bodies. So you need an actual implementation, e.g. include the Glassfish JPA implementation in the classpath and it should work. If you use maven, the following snippet may help:
Beware to keep the above order so that the Glassfish jar comes first in the classpath. Otherwise it will not work either.