I think you need to reference an ITable object from the Many-To-Many RelationshipClass you setup in the Geodatabase. From what I understand, if you were to add this relationshipClass to ArcMap for example, it would be added as a table.
You might need to setup an IEnumRelationshipClass to acquire the RC you need? Not sure what you currently have so far, but maybe you can pull some ideas from this older thread: