There is a conflict between MATLAB and OS X policy about where to Psychtoolbox should be installed.  As of now, the PTB installations instructions instruct the Applications folder, wich agrees with neither.  That was partly because we at the NYU CNS dept MATLAB was an application bundle, which discourages access to the MATLAB/toolboxes directory.

The policy should be changed to advocate the MATLAB/toolboxes directory.  Nonethless, for background here is Apple's policy for OS X.  

Allen Ingling
28 October 2004

from Inside MacOSX: System Overview

Application Support Third-party plug-ins, helper applications, templates, and
other resources for a specific application in a domain. By
convention, these items should be put in a subdirectory
named according to the application. Thus third-party
resources for the application MyApp would go in
Application Support/MyApp. Note that resources created
by the developer of an application should go in the
application package itself. See ТApplication PackagingУ
(page 117) for more information.

Where to install the Psychoolbox ?  /Library/Application Support looks like a good place.  Partly because it solves the problem of specifying to mex files where to find dynamic libs.  Though, as with PsychHID, in some cases we can get around that by compiling in the static version of the lib.  

Downsides are that MATLAB will not automatically include this on its "default" path and that it conflicts with the Windows installation location.  

And if MATLAB (wich it can't) can not handle file paths with spaces then this would not work at all.