Is this a new site (from scratch?). Or are you importing/upgrading an existing site?
Drupal 6 will detect if your server can do MySqli and use it if it can. If it doesn't detect that, it will just give a MySql option on install. Recent versions of MAMP will do both...I normally use mysqli. If yours isn't maybe you haven't updated MAMP in a while.
If it's an existing site, go into your settings.php and change your database designation line to mysql instead of mysqli if it's necessary. Everything else will work as expected.
I am trying to install a local copy of Drupal using MAMP. I set up the database the way that I did with Drupal 5, but when setting up Drupal 6, it wants a MYSQLI database, when MAMP lets you set up a MYSQL. If I try to set up the database in Drupal anyway, it just reloads the page without letting my get anywhere. Help?