Hi,
i searched the web and Apple but found no documentation material about smart folder’s xml list construction.
Some lines are obscure for me, so I want to descend more in depth into the argument (the metadataAttributes document helps me no further).
Hello!
As far as I can remember, it is tricky to program smart folders!
If you open the xml file as a text file, and at the same time look at some documentation for mdfind on the net, it may give you some ideas of how to build up the queries. (This is after memory).
I tend to build the smart folders now adays, normally, and then right click on them in the Finder sidebar to edit them. 
You should be able to find some posts about it here. Here is something I wrote, that I probably should finish!
Edit
It may also help to read about programming Spotlight in general.