From the 'Model' menus you select 'Change Model Set' to bring in a new
model set, then proceed normally.
If your model set didn't appear in the choices, please review the
Customizer instructions for copying over a model set.
If hitting the random button does nothing even with the default mesh
(skin_hi.tri etc.) then you have probably deleted the related
statistics files, 'skin_hi.egm' and 'skin_hi.egt'.
Best wishes,