Are you trying to create a query in the SMS Admin Console, or are you trying to run a query against thw WMI layer?
If you are in the admin console and the fields are simply not listed in the drop down boxes, then either your MOF didn't work, or you need to choose a different base class in the very first "Create Query" dialog box.
If you are trying to script a query (essentially Ad-Hoc), then you should use WMI CIM Studio to browse the WMI namespace on the server to find the tables and see if there is any valid data being reported.
When editing the SMS_Def.MOF file, you should ALWAYS test the MOF file by running (in a dos window) "MOFCOMP.EXE SMS_Def.MOF". This will tell you if there are syntax errors. If there are syntax errors, then the data isn't getting retrieved from the client.
Overall, I would say you should figure out if it's breaking at the client (use MOFCOMP.exe), or if it's breaking at the server (use WMI CIM Studio).