the most common issue with Mamp is that the root account is configured without a password.
Power gallery will not work on servers where the root password is blank.
See the following documentation from the Mamp site for changing the root password:
faq.html#q9
In the wizard, you should be entering "localhost" as the MySQL server name, it sounds like you are entering "/Applications/MAMP/tmp/mysql/mysql.sock" instead.