There are a few ways to do this.

If the piano software is a large program and you want it on another computer, then you could use MIDI cables between the two computers to have one 'play' the other and use an audio cable back from the piano computer to the line in of the recording computer.

Much easier to try installing on the same system and see how it runs.
Install the piano software, remember where you install it.
In PTP select a MIDI track
Click the arrow to the left of the track name (in the main Tracks window) and cleck edit VSTi
In the resulting window is a box with the name of the track and four empty dropdown slots below the name.

Use the first (top one of the four) dropdown and scroll all the way to the bottom and select 'add VSTi'.
Find your already installed piano software, you'll need the .dll file or an .ax file. Most likely the .dll file.
Click OK
Now the information (or notes) on that track will play thru the new piano.

I do not work here, but the benefits are still awesome
Make your sound your own!