If the document building process leads to the need for schema modification, click on the Edit Schema button on the main toolbar (Figure A) to have XML Instance automatically load your schema into the schema editor of your choice (as set in your preferences).
Figure A - Edit schema button on main toolbar
If you do not have a schema editor installed, XML schema dialects (.xdr, .xsd, .sox, .biz) can be loaded directly within XML Instance. After making and saving changes to your schema, hit the reparse button in the XML Instance source view to update the schema rules within XML Instance.