What may be happening with the DB not showing in phpMyAdmin running under MAMP is that you have a manually installed mysql and a different version installed automatically by MAMP?
That MAMP welcome page also had some MySQL default details listed. Try using those in your CI config file, i.e.
Host: localhost
U: root
P: root
Also import your SQL file using phpMyAdmin, so the DB is installed on the same version/location of MySQL which MAMP uses when looking for it.
