What happens when you go to in Aard Dictionary to Dictionary > Add Dictionaries or Dictionary > Add Directory? System's file open dialog should appear and allow you to navigate your device's file system. If your card is recognized by device - then it's there. Now, the open file dialog I get on N810 is not a proper Maemo file dialog but rather a generic one, so internal and external memory show up as "mmc-undefined-name-internal" (/media/mmc2) and "mmc-undefined-name" (/media/mmc1). I'm not sure if on N900 you get something like this or a proper system file open dialog.