From the pine main menu, hit 'S' for Setup and 'C' for Config. Use the down-arrow to move down to inbox-path and hit 'C' to Change the value to this:
{mail.mac.com/ssl/novalidate-cert/user=username}INBOXOf course change username to your mac.com email address username.
When you start up pine, it will ask you for your password, except that you'll probably find that half the time pine gets some kind of SSL negotiation error, and you have to restart it.
Theoretically you ought to be able to use the Folder Collections ('S','L', from the main menu) and not muck about with your primary inbox, but I had much less success with that.

