To use an Astell and Kern device with Dapper you need to have the device show up as a disk on the Mac side. Unfortunately untill Astell and Kern do this themselves, youll need another piece of software called SyncMate pro. You can download this at www.syncmate.com.
Install SyncMate, upgrade to the Pro version, and then add your DAP as shown below.
Its always a good idea to restart the Mac when youre done :-)