I downloaded 3.5.1 and put it on MAMP PRO.
I'm trying to search and install themes and plugins. But it prompts for ftp details.
The root is in /Applications/MAMP/htdocs/wordpress.
The file permissions on this folder are 777.
The file owner on this folder is tim:admin (I'm Tim, the default group is called "admin"). I tried setting it to "www:admin" (www is the user mamp pro seems to use) but that did nothing to help.
MacOS doesn't have FTP built in anymore (easily), and you can't (easily) configure what IS there to use a virtual apache host in MAMP. But the whole "wordpress" folder and all its children are world write so WordPress should be able to do what it likes. But it can't.
there are tutorials around to show 5 year old versions of MacOS how to work on localhost. there are posts here on wordpress.org where the "answer" is to "just use a cheap web host and forget working on localhost", which is absurd.