Sorry, I haven't got much more for you, it works fine here. Only thing I can think of, if you save or render the .mxs from Studio, you'll lose the region...you need to render the .mxs directly with mxcl.exe. But that's about it...just make sure to check that everything else is being written correctly.
Some code might help if you still can't get it working.
[edit: one more thing, make sure you're writing at least one camera into the scene, and that maxwell->setResolution( x, y ) is larger than the region you're defining.]