You are specifying an SQL string which EclipseLink then tries to execute, which isn't the correct way of executing a stored procedure on Postgres. You should create a StoredProcedureCall, which EclipseLink will then form into SQL for the Postgres database if the app is configured to use the PostgreSQLPlatform: http://wiki.eclipse.org/EclipseLink/Examples/JPA/CRUDStoredProcedures has an example.
The example did not show static values - it was defining which fields the parameter value would correspond to. The values get pulled from the objects used in the CRUD operations. So
call.addArgument("P_EMP_ID", "EMP_ID") causes the "P_EMP_ID" call parameter to get passed the value from the "EMP_ID" mapping within the entity.