JPA 2.0 many-to-many with extra column

I'm trying to do a ManyToMany relationship in JPA 2.0 (JBoss 7.1.1) with an extra column (in bold, below) in the relationship, like:

Employer           EmployerDeliveryAgent             DeliveryAgent
(id,...)   (employer_id, deliveryAgent_id, **ref**)  (id,...)

I wouldn't like to have duplicate attributes, so I would like to apply the second solution presented in http://giannigar.wordpress.com/2009/09/04/mapping-a-many-to-many-join-table-with-extra-column-using-jpa/ . But I can't get it to work, I get several errors like:

1. Embedded ID class should not contain relationship mappings (in fact the spec says so);
2. In attribute 'employerDeliveryAgent', the "mapped by" value 'pk.deliveryAgent' cannot be resolved to an attribute on the target entity;
3. In attribute 'employerDeliveryAgent', the "mapped by" value 'pk.employer' cannot be resolved to an attribute on the target entity;
4. Persistent type of override attribute "pk.deliveryAgent" cannot be resolved;
5. Persistent type of override attribute "pk.employer" cannot be resolved;

Many people on that link said that it worked fine, so I suppose something is different in my environment, perhaps JPA or Hibernate version. So my question is: how do I achieve such scenario with JPA 2.0 (Jboss 7.1.1 / using Hibernate as JPA implementation)? And to complement that question: should I avoid using composite keys and instead use plain generated id and a unique constraint?

Thanks in advance.

Obs.: I didn't copy my source code here because it is essentially a copy of the one at the link above, just with different classes and attributes names, so I guess it is not necessary.

First of all You need to generate a EmployerDeliveryAgentPK class because It has a multiple PK:

@Embeddable
public class EmployerDeliveryAgentPK implements Serializable {

    @Column(name = "EMPLOYER_ID")
    private Long employer_id;

    @Column(name = "DELIVERY_AGENT_ID")
    private Long deliveryAgent_id;
}

Next, You need to create a EmployerDeliveryAgent class. This class represent the relation many to many between Employer and DeliveryAgent:

@Entity
@Table(name = "EmployerDeliveryAgent")
public class EmployerDeliveryAgent implements Serializable {

    @EmbeddedId
    private EmployerDeliveryAgentPK id;

    @ManyToOne
    @MapsId("employer_id") //This is the name of attr in EmployerDeliveryAgentPK class
    @JoinColumn(name = "EMPLOYER_ID")
    private Employer employer;

    @ManyToOne
    @MapsId("deliveryAgent_id")
    @JoinColumn(name = "DELIVERY_AGENT_ID")
    private DeliveryAgent deliveryAgent;    
}

After that, in Employer class You need to add:

    @OneToMany(mappedBy = "deliveryAgent")
    private Set<EmployerDeliveryAgent> employerDeliveryAgent = new HashSet<EmployerDeliveryAgent>();

And in DeliveryAgent class You need to add:

    @OneToMany(mappedBy = "employer")
    private Set<EmployerDeliveryAgent> employer = new HashSet<EmployerDeliveryAgent>();

This is all! Good luck!!