If you run SHOW VARIABLES from a mysql console you can look for basedir.
When I run the following:
mysql> SHOW VARIABLES WHERE `Variable_name` = 'basedir';
on my system I get /usr/local/mysql as the Value returned.
(I am not using MAMP - I installed MySQL with homebrew.
mysqldon my machine is in /usr/local/mysql/bin so the basedir is where most everything will be installed to.
Also util:
mysql> SHOW VARIABLES WHERE `Variable_name` = 'datadir';
To find where the DBs are stored.
For more: http://dev.mysql.com/doc/refman/5.0/en/show-variables.html
and http://dev.mysql.com/doc/refman/5.0/en/server-options.html#option_mysqld_basedir