How can I transfer a Large (450Mb) file between two Mountain Lion Macbook Pros?
After enabling File Sharing on one machine, it will give you the name of the machine as used to connect to it. Next, go to the other computer, and open Finder.  You should see the name of the other computer in the sidebar.  If you don't, then press Go -> Connect to Server, and connect to the other machine by the name that was given in the Sharing window.

When you connect, you should see the public folder of your user account, and the Drop Box folder in there.  Copy the file to the Drop Box folder by dragging it onto the Drop Box folder.  On the target computer, open Finder, your user folder, the "Public" folder, and the "Drop Box" folder, and you will see the file that you copied.