Voicebanks made for Windows will also work on Mac. When you download them, be sure to unzip the files using Unarchiver to keep the file names intact:
https://theunarchiver.com/
If at any point you're prompted to select a character encoding, choose Shift-JIS to preserve Japanese characters.
Once you have a plain voicebank folder, change the name to have ".utau" at the end and you will be able to double click it to open in UTAU-Synth.
For basic troubleshooting, you might find it easier to join a Discord server where you can chat with people in real time. I have a link to St. Defoko's School of UTAU in my signature on this post. But always feel free to come back to UTAforum to share what you create!