Rock Band 3 keyboard and GarageBand not working - help!

i'm having trouble using my Rock Band 3 keyboard as a MIDI input device to my Mac Pro (Lion) running GarageBand.

i'm using a generic midi-USB converter cable

i connected the keyboard & computer, plugging the midi IN plug of the cable into the midi OUT socket of the keyboard.

RESULT:

RB3 keyboard: all 4 red lights on the keyboard flashes

Midi-USB converter cable: red power LED = on, amber "IN" LED flashes about 3x per second

GB: program does 'detect Midi device'

BUT, pressing keys on keyboard has no effect, and there is no indicator on bottom blue display panel of GB when i press a key.

PS: if i REVERSE the connection, just in case teh cable was wired wrong (plug midi OUT cable into midi OUT socket), it still doesn't work, but 2 differences:

RB3 keyboard: a single red led comes on (opposed to all 4 red lights on the keyboard flashing)

Midi-USB converter cable: red power LED = on, IN and OUT LEDs stays off

help!

thanks!