Not all of the included login screen choices show the list of users. If the one that you are currently using does not, you will have to enter your user name manually. If, after ensuring that you have the correct user name and password, you are still unable to log in, please make sure that your Caps Lock key is not enabled.
If you are still unable to log in and it is your computer or one that you have permission to access, you may wish to perform a forum search for lost password and/or lost user id. There are threads that describe changing them, IIRC.
Mint 17.1 Xfce 4.12.
If the fact that my post is longer than a standard SMS message causes you to be unable to take meaning from it, do not blame me for something that is either the fact of the school teachers you had or the sludge between your ears.