This is really a server question and not specific to WordPress.

Be sure mod_rewrite is enabled in Apache on your version of OS X. Google that. Restart Apache after making changes to httpd.conf.

You may also need to add a blank .htaccess file in the /Users/yourusername/Sites folder so that WordPress can write to it.

This is really a server question and not specific to WordPress.

Be sure mod_rewrite is enabled in httpd.conf Apache on your version of OS X. (Google that with your version of OS X.) Restart Apache after making changes to httpd.conf.

You may also need to add a blank .htaccess file in the /Users/yourusername/Sites folder so the file has the correct permissions and so WordPress can write to it from the Settings >> Permalinks screen in admin. And to do that, you will need to add the .htaccess file in Terminal, as OS X hides unix "invisible" files - files with a leading . - in the finder.