I'll give it a try.
First of all, to use your keyboard's speakers, the keyboard MUST have AUDIO IN. This is usually an RCA plug, round and 1/4" diameter (and usually in pairs for left and right). If you don't have this, then the speakers on the piano cannot be used.
However, keyboard speakers aren't usually very good anyway.
Your best option is to use headphones for two reasons:
1) they will sound better than the speakers on the keyboard,
2) these can plug into your soundcard or the computer soundcard.
If you can get Pianoteq sound from the computer speakers, then you can plug headphones into the headphone jack - this will likely be a mini RCA connector (2.5mm I believe).
Note that the soundcards in computers are not very good. For the best sound from Pianoteq, a separate soundcard is essential.
There are some pretty decent soundcards from Sound Blaster at reasonable prices. These aren't high end by any means, but much better than the soundcard on a computer. My EMU 1820M broke down last year, and while it was being repaired I resorted to the computer soundcard. It was to terrible that I couldn't do any music until the EMU was fixed.
BTW, I looked at the onsite picture of the backside of your keyboard, and it doesn't appear to have AUDIO IN connectors.
Hope this helps.
Glenn
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Procrastination Week has been postponed. Again.