Go to xampp/xamapfiles and right click on htdocs and choose "get info". Use the "details" drop-down arrow at bottom. Change the permissions to read write for everyone. You may have to "unlock" it. You probably want to 'apply to enclosed items" as well. Let us know if that works.
I assume you are running you Mac as an "admin" user? If not, that could also be your problem.
The XAMPP desigers over-engineered the security of the system in my opinion. I would have made it more user-open since it is designed to be a desktop development system and not for a local intranet or the world-wide internet.