First, try using the GPX Driving Directions web service directly. Maemo Mapper uses this web service for downloading directions.

If your query still doesn't work, try doing the exact same query on http://maps.google.com (use the "Get Directions" button). The web service works in the same way as Google Maps, and it should work with the same query text. If your query still doesn't work, then there is something wrong with your query.