Set file permissions
On Mac OS X (Leopard), the Apache HTTP Server runs under the user account, _www which belongs to the group _www. To allow WordPress to configure wp-config.php during installation, update files during upgrades, and update the .htaccess file for pretty permalinks, give the server write permission on the files.
One way to do this is to change the owner of the wordpress directory and its contents to _www. Keep the group as staff, a group to which your user account belongs and give write permissions to the group.
$ cd /<wherever>/Sites/<thesite>
$ sudo chown -R _www wordpress
$ sudo chmod -R g+w wordpress
This way, the WordPress directories have a permission level of 775 and files have a permission level of 664. No file nor directory is world-writeable.
You can read more about it on mamp forum: http://forum.mamp.info/viewtopic.php?f=2&t=15917