I know this post is 18 months old, but I was having the same problem (could not figure out how to change permissions for my whole directory on Mac OSX). Finally found the solution on the stackoverflow page on the "getting started" article, however the solution for me was not the one linked to, but the one posted by a Bryan Kimani:
1Under General Tab, in XAMPP app, click Open Terminal
2 A terminal will be launched with something like, root@debian:~#, on the terminal shell
3 on that terminal shell, type, chmod -R 0777 /opt/lampp/htdocs/ and enter
4 Exit, the terminal and you be good to go