I do not know if this will help, but I also use MAC and XAMPP.
My solution was to give full permissions to the folder prior to "Install", I mean the folder that contains it in my case my "PrestaShop" folder.
I right-click this folder, click on "Get information", then in permissions I have added all possible users "admin", "_mysql", "user (me)", "everyone".
Then I have unlocked the small yellow padlock, and click on the small gear wheel, and click "Apply to the included items..." This is going to apply the changes to all the folders inside your PrestaShop folder. Then you have to be able to delete it.
Hope this work for you.
Also, I'm having a problem to access to my admin page that can´t resolve hope someone here could help.
Regards!
PS. Tell me if you misunderstand something.