criteria.Add(Expression.Eq("User", userID));
criteria.Add(Expression.Eq("Meeting", meetingID));

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="DataAccessComponents.Entities" assembly="DataAccessComponents">
   <class name="UserMeetingRoleEntity" table="[UserMeetingRole]" lazy="true">
      
      <id name="Id" column="ID" type="Int32" unsaved-value="0">
         <generator class="native"/>
      </id>
      <many-to-one column="[UserID]" name="User" class="UserEntity" />
      <many-to-one column="[MeetingID]" name="Meeting"  class="MeetingEntity" />
      <property column="[Role]" type="Byte" name="Role" not-null="true" />
      
   </class>
</hibernate-mapping>

        public static UserRole GetByUserAndMeeting(Int32 userID, Int32 meetingID)
        {
            ISession session = SessionProvider.Instance.GetSession();

            try
            {
                ICriteria criteria = session.CreateCriteria(typeof(UserMeetingRoleEntity));
                criteria.Add(Expression.Eq("User", userID));
                criteria.Add(Expression.Eq("Meeting", meetingID));

                IList<UserMeetingRoleEntity> meetingRolesList = criteria.List<UserMeetingRoleEntity>();

                if (meetingRolesList != null && meetingRolesList.Count == 1)
                {
                    return (UserRole)meetingRolesList[0].Role;
                }
                else
                {
                    return UserRole.Unknown;
                }
            }
            catch
            {
                return UserRole.Unknown;
            }
            finally
            {
                SessionProvider.Instance.CloseSession();
            }
        }

IList<UserMeetingRoleEntity> meetingRolesList = criteria.List<UserMeetingRoleEntity>();

could not execute query[SQL: SELECT this_.ID as ID1_1_, this_.[UserID] as column2_1_1_, this_.[MeetingID] as column3_1_1_, this_.[Role] as column4_1_1_, userentity2_.ID as ID0_0_, userentity2_.[Email] as column2_0_0_, userentity2_.[Password] as column3_0_0_, userentity2_.[Type] as column4_0_0_, userentity2_.[Preferences] as column5_0_0_ FROM [UserMeetingRole] this_ left outer join [User] userentity2_ on this_.[UserID]=userentity2_.ID WHERE this_.[UserID] = ? and this_.[MeetingID] = ?]

{"There is a problem with your mappings.  You are probably trying to map a System.ValueType to a <class> which NHibernate does not allow or you are incorrectly using the IDictionary that is mapped to a <set>.  \n\nA ValueType (System.Int32) can not be used with IdentityKey.  The thread at google has a good description about what happens with boxing and unboxing ValueTypes and why they can not be used as an IdentityKey: http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=bds2rm%24ruc%241%40charly.heeg.de&rnum=1&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26q%3DSystem.Runtime.CompilerServices.RuntimeHelpers.GetHashCode%26sa%3DN%26tab%3Dwg\r\nParameter name: key"}