*IF* I understand the question (more than likely, you should consult an AE guru at your site/company, since there are usually server-side, publishing engine, and permissions issues -- and I'm clearly NOT that guy/gal, nor have I that kind of knowledge, but here are some ideas that may help):
From the Army Aviation DTD/FOSI download site, be sure to download the "supplemental" document as well. It mentions what needs to go into folders and what files you may need to edit.
From Arbortext Editor, Tools --> Preferences --> File Locations (following the Army Aviation site supp. doc), add the appropriate local pointers to "custom\doctype" etc. folders. You may need to edit some of the stuff you download from the Army site, depending on what type of doc's you are creating (dash10, MTF, etc.)
Then, once you have everything in place in the folders and are pointed to properly from AE (edited as needed, with proper permissions):
Go to AE's File --> New and select "Add." Select the DTD you need. It will then appear on the "Document Type" list.
I hope this helps. IMHE, AE has a lot of config issues that a sys admin type of person usually helps set up.
Best wishes!