PolySynth is actually a completely new algorithm, not just a preset.
On the pedal itself, first make sure you’re running the correct firmware: go to System (press the PROGRAMS and ROUTING buttons together)->>About, and see if the software is the official 1.6.11 or the new 1.7.1 beta.
Next, press the PRESETS button, set Search to All, Type to Synth and Algorithm to PolySynth. Rotate the Select knob to scroll through the different PolySynth presets.