All of the NORMA schema files are installed with NORMA. If you open any .orm file using the XML editor, right click on an element in the XML and select 'Go To Definition' you'll jump into ORM2Core.xsd.
The ORM2CoreXsd.htm file in NORMAINSTALL\Documentation is generated from the core xsd file and provides a more navigable form of the schema file.
PS To make it easier to switch from designer to XML editors for the file, select the Environment/Documents page in the Tools/Options dialog, turn on the 'Show Miscellaneous files in Solution Explorer' option and set the 'items saved' value to something other than zero. You can now open the .orm file normally, close it, and still see the file in the Solution Explorer tool window. Right-click gives you an open-with option to see the XML.