You need to tell DITA2Go where to find the DTD for your document only if you are not using catalogs, and your document includes specializations. If the SystemID in the doctype declaration for your DITA XML document does not designate an accessible local “file:” URL, you must specify a local path to the DTD.
To tell DITA2Go where to find the DTD for your project:
[Setup] ; DTDPath = local path to use for DITA DTDs if the SystemID in the ; doctype declaration is not an accessible local "file:" URL DTDPath=%OMSYSHOME%\d2g\dtds\dita1.1
A copy of the Oasis DITA 1.1 DTD is included in your DITA2Go distribution, in directory %omsyshome%\common\dtds\dita1.1. If you have specializations, you must use your own DTDs instead. Or if you already have your own copies of the Oasis DTDs elsewhere for use with other applications, you can use those instead.