Dear Berna,
thank you for sharing the details of your issue with the ATK community.
Classical potentials such as those you want to used are not only used in the Interface Builder, which I guess from your question is the VNL plugin you are actually using. They are actually mostly used in ATK as a calculator and as such they can be selected in the Script Generator.
Unfortunately, if the shift-surface button is not active, it means that there is no classical potential in ATK available
for your desired combination of elements (in this case, Zn, O, Au). If you have your own potential, you can actually implement it as specified in:
http://docs.quantumwise.com/tutorials/combining_potentials/combining_potentials.html.
Another, probably easier option is to shift the surface manually and set the distance using chemical intuition: for most system, bonding distances are in the range 2.0-3.5 Angstrom, so setting a distance in this range will probably provide a reasonable starting guess. Then, you can optimize the structure of the interface using DFT, which is also implemented in ATK. Although I am not an expert, I am aware that the ZnO-Au interface has been studied a lot, so I think you can also check the literature to obtain reasonable estimates for the correct binding distance.
Regards,
Daniele.