You can send and recieve on any of the 16 channels on the WK. All you have to do is match up the midi channels on both units. However I can't see the DD55 having multiple midi channels, but it may have.
I would assume this is what you'd do. Connect a midi cable from the out port of the DD55 to the in port of the Casio. If the DD55 just has basic in and out then that should work. pressing the start button on your DD55 should trigger the Casio. I don't have a DD55, but I have several drum machines. You'll need to look into your manual and read the section on midi that deals with using the DD55 to (trigger) another midi device. They're usually laid out fairly easy to understand in the Yamaha manuals.
[This message has been edited by squeak_D (edited 12-13-2005).]
GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.