Here’s a link to using the Sims 4 Tray Importer in macOS. Getting it to run seems a bit hit or miss but it appears that with a little perseverance some users are getting it to run in Sierra.
The guide is really detailed and very easy to follow and the comments are very informative too – good luck!
Update January 2018
As of January 2018 the Mac version of Tray Importer is no longer compatible with the Sims 4:
S4TI for MAC is no longer available. The app no longer works with the latest update of the Sims 4 and the developer does not want to provide support on MAC (platform it does not have and which greatly complicates its development …)
Sorry folks, seems like it’s the end of the road for us Mac users. I’ll update this page if it is resurrected.
Update April 2018
Well, seems like it’s up and running again as of the 4th April 2018. Just follow the links above!