See my dba.stackexchange.com answer: How to restore root to MySQL server on MAMP

Assuming you went with a generic install of MySQL, my procedure for MAMP is:

1. Edit your my.cnf, probably located at: /usr/local/my.cnf, adding the lines:

skip-grant-tables
skip-networking

2. Restart mysqld

sudo su -
/usr/local/mysql/bin/mysqladmin shutdown
nohup /usr/local/mysql/bin/mysqld_safe &
exit

3. Change root privs

mysql> GRANT ALL PRIVILEGES ON *.* TO root@localhost IDENTIFIED BY 'password' WITH GRANT OPTION;

4. Remove the skip lines from the my.cnf

## skip-grant-tables
## skip-networking

5. Restart mysqld again

sudo su -
/usr/local/mysql/bin/mysqladmin shutdown
nohup /usr/local/mysql/bin/mysqld_safe &
exit