must have a primary key on table when inserting ?

You don't need to modify the database at all to make this work.  As David stated, you can form a unique key out of the first two columns in Table_Link - and you can do this directly in the Object Mapper.  DevForce needs every entity to have "identity", but that doesn't mean you must strictly mirror primary keys in the database.  If your table doesn't have a primary key, DevForce, via the Object Mapper, will allow you to specify the columns that DevForce will use to enforce "identity" on the entity.  So in your case, open the Object Mapper, and on the "Simple Properties" tab for this entity check off these two properties as "Key" properties.