We don't currently have a facility to generate the XML structure for you. This would allow you to decide which facts types were mapped to XML and which to relational structures.
The only thing you can do at this point is to essentially hijack another datatype to declare a generated field as XML. See the previous thread http://www.ormfoundation.org/forums/t/601.aspx for directions on adding uniqueidentifier to an alternate generator and follow the links from there. This should at least let you declare an xml data type, even if it doesn't declare any structure for it.
PS The generator registry settings are discussed as being under HKLM\Software\ORM Solutions in the linked responses. However, VS (and NORMA) are 32-bit apps and appear under a registry branch on a 64 bit system, where you need under HKLM\Software\Wow6432Node\ORM Solutions. Also make sure you get your VS version correct.