The user guide mentions the requirement of an omnimark.lib and catalog.xml file with an entry for the DTD/Schema, how do I creates these two files. Adidtionally if the XML being imported is a DITA XML then how can I configure its DTD?
What version of XPP?
Where are you reading about the omnimark.lib and catalog.xml files in the XPP documentation?
You said the User Guide, but in the latest version of the XPP manuals on the web (docs.sdl.com) I only find references to omnimark.lib and catalog.xml in the XML/SGML Reference manual (ml.pdf) and the Transforming Data manual (condata.pdf) - and just a couple of references to catalog.xml in the Command Line Utilities manual (cmdutils.pdf) under the xppxslt program.
I suggest you download from docs.sdl.com the latest version of XML/SGML Reference and read Chapter 1 and Transforming Data and read Chapter 10 about the omnimark.lib and catalog.xml files. I think it's fairly well described (in latest versions of these manuals) how to put in place and use these two files. I don't think I can describe what you need to do better than those chapters do.
If you still have questions after doing that, then please ask.