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.
Proud to be a Mint user running 64-bit Mint 17 XFCE.
"Change should never be your goal but, instead, only a means to reach your goal."