more than likely a simple permissions problem on the directory. in a command prompt, go to the top directory (the place where the directory in question is located) and type ls -l. it should show something like this
drwxrw-rw- root root someFile
d--------- root root dirYouCantSee
if thats the case, then you should change the permissions of that directory (provided that this directory you cant see contains just music, and no program files, libraries or anything else that can be goofed by changing permissions). the best way to do that is use chmod, the linux command line tool. it should look something like this
sudo chmod 644 -r dirYouCantSee
this will change the permissions of the directory and its contents so that you can view them from your media player. cheers!