That's why it doesn't work.
The path shown in the Preferences containing the Arduino folder is telling you that is where it looks for libraries but because you don't have a folder in the Arduino folder named Libraries containing all your installed libraries, the IDE doesn't know where to look for your ws2803 library. You need to cut and paste the entire ws2803 folder into a new folder you create in the arduino folder shown in preferences . All other locations are useless. Only libraries contained in a folder named Libraries in the arduino folder shown in the Preference will work.