I'm having the exact same problem.
It's a bug with the mapping attributes. If you look at the mapping file that the attributes create, you'll notice that there is no <filter-def /> section at all. The <filter-def /> section needs to go under the "root" of the mapping file and in order to do that you'd have to put the [FilterDef] attribute on a class or in it's own file or something. But the [FilterDef] attribute can only go on a property, indexer, or member in the C# code.
It's impossible to put the [FilterDef] attribute in the correct place!
-Tim Gebhardt
tim@gebhardtcomputing.com