Hi Amy...
Well .. you should become familiar with the Structure Application Developers Guide (PDF installed in your FM/docs folder) .. this has all of the info you'll need.
You're on the right track .. it sounds like you're doing the right thing. After using Open DTD, you should end up with an EDD (or the start of one). If there are errors in this import process, you'll need to resolve the errors before you can continue. Many times you'll need to make tweaks to the EDD to get certain element types properly set up (tables are often a problem). You'll also need to make adjustments to the EDD so that certain elements are rendered in a useful way (empty elements may need to be a marker). For a complex DTD, there's often a lot of work to make the needed tweaks. This will also involve setting up a read/write rules file, as well as a template. At a minimum the structure application consists of an EDD, rules file, and template.
It's likely that someone else is also working on developing a structure app for this DTD, you might post a query asking specifically about that.
This process of making a structure app from a DTD can be bit of work, but keep in mind that once you're finished and are able to open the XML files in Frame, you're basically ready to print (assuming you've set up your template with meaningful styles). If you use other XML editors, you'll be able to open the files right away and start editing, but in order to print, you've got to do a lot of work. It's basically a tradeoff .. and from what I've seen the FM approach is typically less work in the long run.
BTW .. where does one find this DTD? I made a quick Google search and found the MIL-STD-40051 specification, but no DTD. I'd be interested in checking this out, can you point me at it?
Cheers,
...scott