Mamp defaults on port 8888 for Apache and 8889 for mysql. But it can also use the default apache 80 and mysql 3306 ports.
For the Automod local ftp settings, they should be like this :
the ip address is 127.0.0.1 (that's localhost)
the user is your mac user, password.
path : the absolute hd path to your phpbb directory. i'm using Zend pdt, so that's /usr/local/zend/apache2/htdocs/phpbb3
if you use direct mode, and it says direct is not writable, change the permissions to /store/mods to :
and apply to enclosed items.
then just place your unzipped mod in /store/mods and proceed to install.
i always use direct mode on localhost btw, never had issues.