The EJB3 plugin is not meant for AS 4.2.x. Infact, work on EJB3 related code in AS 4.x was stopped long back. There have been numerous changes in EJB3 code in AS-5 and the plugin is applicable only to AS-5 versions.
As for the specific issue, the options you have is to upgrade to AS-5.1.0 and use the plugin or fix the code by downloading the AS source code. I would recommend upgrading to AS-5.1.0