With the VGP player, what worked for me was to set the number of Reserved Voices in the Options tag, and to confirm it with the "Apply" button (that really belongs to the DFD section above it!). Then you can set the voice count for the actual preset in the VGP main interface (just above the horizontal slider to the left).
You must left-click and hold down on the number on the interface while you move the mouse up or down. The mouse works like a slider. (I know this is how it works on a PC - it should be tha same on Mac)
Also, read the paragraphs in the manual about changing polyphony.
The manual can be found by clicking the Kontakt logo on the VGP interface, then the "Additional Info" button in the About Page. Then click the "Virtual Grand Piano Manual" link to the left in the HTML page that opens.