My version of spice didn't come with a pot model either. Put both attached files in "User files" directory and add ".include potentiometer.sub" directive.
On my machine, the folder "C:\users\dan\Documents\LTspice" shown in the screenshot is the virtual drive path for "/home/dan/.wine/drive_c/users/dan/Documents/LTspice". The symbolic link for the path is "/home/dan/Documents/LTspice" which is where I put my files.
I found the error.
I installed LTspice for everyone, so the library needs to be in an other place:
C:\users\bert\Local Settings\Application Data\LTspice