It depends how you set things up. If you just want to use the sounds from your keyboard and sequence them on your pc, you would do it like this: Run the keyboards midi out to the midi in on your pc's midi interface. Set the keyboard and a midi track in cubase to the same midi channel and record your song to cubase. Tweak it if you want in cubase, and then you can play it back using the sounds on your keyboard by sending the midi signal from your pc's midi out into your keyboards midi in (again using the corresponding midi channel). Of course you could also use route the midi track in cubase to a different synth or sampler for playback as well.