it wasn't correct, I'll try to explain it more detailed. Again the property of the DB is useless while manual tag creation, but it doesn't matter. In the WinCC you should create just one tag of the structure type, all the individual tags from the structure with the dots are created automatically.
So on the place where're the other tags as you wrote add a new tag, the name is to be just "Boiler1/FEEDW", in the DataType section select from the list (at the bottom under a standard types) the structure you want (FEEDW) and in the Address section fill in the beginning of the structure in the PLC (DB19,D0.0). After confirmation you'll see all the individual tags created in here, the text internal tags in the Internal tags folder and the structure instance "Boiler1/FEEDW" in the structure tag FEEDW folder.