CodeIgniter Newbie Question

A few things to check:
Did you setup your MAMP correctly?
Is localhost in your hosts file?
Did you set the $config['base_url'] to the correct path in your application/config.php file?
Do your controllers have names with first-caps?
If your form is pointing to a controller named "form", then there must be a method "index" inside it. That's how CI works: if you call a controller without a method name, it assumes you mean the index method. It's better practice to point your form to a controller that has a more meaningful name. In this case Login.php. Create a method ( = function) called "process_form".
In your view, point to this method: echo form_open('login/process_form');

I hope this will help you further.