Step 4-6. Include .mod Declaration Set

Include the domain element declarations. In concept.dtd, find the comment that says "DOMAIN ELEMENT INTEGRATION". Following that comment, add a declaration and reference for the xmlDomain.mod declaration set:
<!ENTITY % xml-d-def     
  SYSTEM  "xmlDomain.mod"

As for the .ent file, the actual system identifier needs to reflect the actual location of your version of xmlDomain.mod.