The current version of MAMP may require some troubleshooting to get Drush to work correctly. Known issues include:
- Default use of php5.3.6 instead of php5.2.17 causes errors upon update, most commonly "Cannot redeclare class" errors. To solve, change the version of PHP in the MAMP preferences panel or add a line in your .bash_profile to tell Drush to use the right version of PHP
- Inability to connect to localhost MySQL via socket. Causes errors like "Error: PDO::__construct():  No such file or directory (trying
to connect via unix:///var/mysql/mysql.sock)". To solve, from within Terminal run the following commands:
sudo mkdir /var/mysql
sudo ln -s /Applications/MAMP/tmp/mysql/mysql.sock /var/mysql/mysql.sock
Or, if this error appears with MAMP4, consider checking the "Make this version available on the command line" option.