For PC/Mac
Zwift workout files are saved under Documents/Zwift/Workouts as “.zwo” files.
If you have downloaded a new workout file and want to use it in Zwift, simply save it to Documents/Zwift/Workouts, then start up Zwift. Your workout will appear under the “Custom Workouts” category in the workouts list.
To further organize your custom workouts you can create directories under Documents/Zwift/Workouts and save your workouts to specific directories.
For iOS
See How to copy workout files to Zwift for iOS.
This is not working at all! Want to do the coaches challenge and having a heck of a time figuring this out