Right-click on the program button, and select 'Load User Wavetable (.wav)' from the dropdown list.
It must be a mono .wav file, of any sample rate, up to 64,000 samples in length. The information can be found in the compiled Release Notes 1.1~1.4.rtf, under z3ta+'s Documentation subfolder. For more techniques, and a process for transforming waveshapes using z3ta+ alone,
see Steal This Waveshape (Part I).