If you want to modify the MySQL configuration in MAMP, just create the my.cnf file by the following command

• cp /Applications/MAMP/Library/share/mysql/my-medium.cnf /Applications/MAMP/Library/my.cnf

Now you can edit the /Applications/MAMP/Library/my.cnf and restart the server for those modifications.

Done =)

Reference: Create MySQL configuration file (my.cnf) on MAMP