In order to log in without an internet connection, the user must have logged in to the Chromebook at least once when it was connected to the Internet. When a new user logs in to a Chromebook, a profile for that user is built in the background and is what allows the user to login when the Internet connection is removed.
Step 1: From the login screen, click the "sign in as an existing user" link:

Step 2: Enter your full email address and log in as you would normally.

You should now be logged in to your local Chromebook profile.