[sparqee]

Just to prevent any confusion; USB is different from Firewire. I don't think he TASCAM US-144 supports Firewire so I'll assume you mean that you have a USB cable connecting the Tascam to your laptop. This USB cable moves digital audio and Midi data back and forth between your laptop and the Tascam. If you use a pair of S/PDIF cables to connect the Yamaha to the Tascam then you should have digital audio coming from the Yamaha into the Tascam (and therefore into your laptop via the USB cable). The way to test this is as follows:
1. Connect everything as mentioned above
2. Plug some headphones into the Tascam
3. Turn the Tascam monitor mix knob to 12 o'clock
4. Turn up the Tascam "Phones" knob
5. Play the keyboard

You should hear the sound of the Yamaha through the headphones. If this works then you have digital audio going from the Yamaha to the Tascam via the S/PDIF cables. If you don't hear the Yamaha then you may need to take a look at the output settings of the Yamaha to see if S/PDIF needs to be turned on or if audio needs to be routed to that interface or something (sorry, don't know the details of how the Yamaha works).

Once you've got digital audio flowing from the Yamaha to the Tascam you need to see if it's getting to your laptop. Fire up Cubase, add an audio track and activate that track's monitor button (see the Cubase manual if you're not familiar with tracks and monitoring).

Once you've got sound going all the way through from the Yamaha to your laptop you've got to get the laptop to send control data to the Yamaha. If you just want to use midi you can connect the Yamaha to the Tascam with a pair of midi cables, but if you want to use the fancy VST support between Cubase & the Yamaha then I suspect that you will need to connect the Yamaha to your laptop with a USB cable. Hopefully your laptop has more than one USB port. I wouldn't recommend putting the Tascam on a USB hub since that might degrade your data throughput but if you only have one USB port on your laptop then give it a try.

I'm also guessing that you need to install some kind of Yamaha software (i.e. a VST plugin) to your laptop in order to get the Yamaha to show up in Cubase as a plugin.