In the Mate Login screen did you press the middle button .. see picture here:http://www.webupd8.org/2012/05/install- ... dm-in.html
that looks like a window?
Basically type in your username but do not press enter. Press the middle button, select the session you want, and then press enter in the textbox. Finally enter the password. You will get a dialog asking if you want to change session just the one time, or you want to change the session permanently.
Alternatively you can try changing the theme:
Linux Mint offers you a Mint Display Manager (MDM) to configure a login session. You can use it to change a login screen or a theme you like in a few steps:
Go to Menu > Administration > "Login Window" to open this window "Login Window Preferences".
Under the "Local" tab, select a built-in theme such as "Circles", then log out and back in to try a new log-in screen.
Another riskier alternative is that you can change back to lightdm (which ubuntu uses):http://www.webupd8.org/2012/06/how-to-u ... dm-in.html