Hint: DON'T DO THAT.

IMHO, the only reason to have a TR-909 is for the full package - the sequencer and the sounds. If you're going to sequence it from Cubase, I would recommend you get a VSTi sampler instead and load it full of one of the many well-made TR-909 sample kits out there.

Seriously, the magic of having a real TR-909 is so that you DON'T have to use the computer! Learn the sequencer, and *play* with it!

As for your actualy problem - are you sending on the same MIDI channel as the 909 is set to?