Were currently using MIL-STD 40051-1C to develop a new IETM, we're having difficulty applying the XSL stylesheets to display the published style. Here's a link to website that has our official 40051C_v6_4 stylesheets for reference.
I think you may be misunderstanding the way that the MIL-STD 40051-1C stylesheets are intended to be used. I had a quick look at the XSL stylesheets in the cstyle-v6_4 folder and these are XSL-FO stylesheets probably designed for use with Arbortext Editor or related applications. Theoretically it would be possible to format the output from FrameMaker using these stylesheets, but I couldn't be certain without trying it myself.
With FrameMaker the missing part of the process would be an XSL:FO processor that would use the xsl:fo formatting objects to deliver a ready to use PDF or some other output. FrameMaker's built-in formatting mechanism (EDD) is nothing like XSL:FO in concept or scope.
Having said all of that it woud be possible to trigger the XSL:FO processing from FrameMaker by including a "post process" XSL reference in a structured application definition. The output from FrameMaker would be an XSL-FO file that could be fed into a suitable FO processor...
It's not quite that simple. Although many of FrameMaker's concepts are identical between usntructured and structured, the correct definition of structured document formatting does require an understanding of FrameMaker structure application development. If you already have an unstructured template that has the correct formatting it is possible to make use of the existing paragraph and character formats in the EDD. The EDD or Element Definition Document contains the structure defintion (just like an XML DTD) along with the context aware formatting rules.
I can't really go through the whole structure application development process right now, but you should be able to get a lot of help on this forum and of course in the User Guide and Structure Application Develper Guide and Reference manuals.