To install a sim you must unzip the file and place only the unzipped files in your tray folder, not the zip file/folder. The location of the folder is:
Documents/Electronic Arts/The Sims 4/Tray
The unzipped folder should contain 5 files with random numbers/characters in front. Do not rename them.
.householdbinary
.trayitem
.sgi
.hhi
.hhi
The downloaded sim should appear in your personal gallery.