FrameMaker does not really work with DTDs but they have their own format (EDD = Element Definition Documents). These are F+S files that have rules analogous to DTDs plus lots of additional stuff like formatting rules. So a EDD covers what does DTD + XSL/FO + applications in XML world ...
Now you can load the DTD you did in See Fix your XML DTD.
Here is an example, the following XML DTD entry:
<!ENTITY % Standard-attrs "Id ID #IMPLIED"><!ELEMENT Stepbystep ( Doctitle, Info?, Abstract?, Intro?, Prereq?, Steps+, Comments?, Conclusion?, Open? )><!ATTLIST Stepbystep %Standard-attrs; >
Element (Container): Stepbystep Valid as the highest-level element. General rule: Doctitle, Info?, Abstract?, Intro?, Prereq?, Steps+, Comments?, Conclusion?, Open? Attribute list Name: Id Unique ID Optional
Usually, you don't need to do this, but if you imported a DTD without having defined a SGML application for it, you can catch up a little. The beginning of your EDD file must have something like this: