For the inexperienced IT support person it is going to be quite tough to figure out what is wrong with the Schema or DTD file. That file is essentially the set of rules governing the allowable document structure e.g. what tags are allowed to go where, such as enforcing that each chapter can have only a single title.
What you're meant to do with Arbortext is install "doctypes" (document type configurations) which combine the Schema/DTD, stylesheet (for screen or print styling), scripting for any customisations, etc. Arbortext comes with a number of doctypes out-of-the-box - if you go File->New you will see a range to select from.
If the user's XML does not match an out-of-the-box doctype then really you should create a doctype plugin to support it. The doctype plugin would contain the Schema/DTD files, XML catalog file, and .dcf file. That is basically the minimum. This plugin can then be placed in a "custom" folder and linked into Arbortext using the APTCUSTOM environment variable to point to the plugin path.
Those are the basics. In many cases an experienced third-party services company such as GPSL (www.gpsl.co) is better placed to help create and support doctype plugins.
