A private folder in a package works fine in my Matlab 2013a.
What doesn't work, however is non-private package subfolders.
That is, I have a package +pack and inside this package I have a function 'packfun.n' and two folders: 'private' and 'notprivate'.
The function 'privfun.m' from the 'private' folder is accessible. Whereas the function 'nonpriv.m' from the 'nonprivate' folder I didn't manage to run no matter how I tried.