1. You boot a Mac.
2. It presents you with a list of user icons.
3. There is no password box.
4. What do you do when you have no mouse/trackpad?

Answer:

At login there is a list of names (if multiple people are set to use the computer) and guest, type the first letter of the name that you want to login with, then hit enter. You are now able to enter your password. i.e. if your name is Sam Jones press s then ENTER and you are ready to go.