I created a "doctypes" folder in "my Arbortext" folder on my hard drive, outside of the PTC (Arbortext Editor) folder. I downloaded my DTD from the AMECOM website and inserted the DTD in the doctypes folder (the other files you see in "my Arbortext" folder were also downloaded from the same site as the DTD).
I open (File->New) a new document in Arbortext Editor and the New Document window appears. I select the browse button and navigate to my doctypes folder, select the DTD I need to use (as indicated-You will navigate to where your DTDs are stored) and then my new document using the selected DTD opens and I can start working. Hope this helps.
Mike
