Need help hooking up Oxygen8 w/Cubase SE

[ednor59]

Man, I am at the end of my ROPE here!
Here's the rig: Mac PowerBook G4, 1.33gHz, 2gb RAM, OSX 10.4, Oxygen8 v.2, Tascam US-428.
I've got a separate power supply for the Oxygen8, so I don't need to plug it in via USB. Got it plugged in to the MIDI input of the Tascam. When I push the keys, the MIDI light on the Tascam responds, so that connection is there.
I have read the manuals for Cubase SE, the Tascam, and the Oxygen8, and I'm stuck.
The Oxygen8 works on its own using Ableton Live Lite that came bundled with it. But I've been recording for three years using Cubase, and I'm determined to get this thing working with my existing rig.
I've downloaded and installed the latest driver from Steinberg.
I'm a guitarist, not a keyboard player, but I just want to add a few chords of organ here and there. Any help or advice anyone could offer will be greatly appreciated. Thanks in advance!