Yes, those instructions are for the retail version not the free one. Just skip it. Update ZA to latest antivirus definitions, reboot the PC in SAFE MODE with NETWORKING. Open manually the ZA interface and from the drop down button "SCAN NOW" (your 2nd screenshot) choose "full scan" (it should be there in the free version).

If you still don't find the way then best is to forget about ZA and get volunteer help by malware experts at bleepingcomputer or spywarehammer forums (link to these forums are contained always in the Malware guidance). They will guide you hand in hand to a full check of the system. This way you will be 100% sure you are clean and free of any malware or leftovers from malware. Next time do not trust shops but go directly to these dedicated forums.

You may also think to pay for antivirus as this will also bring support to it. Most of the time is not just the tool you use but about common sense and few basic actions.

See here for some tips on how to improve protection: