Is it possible to estabilish Dedicated Administrator Connection (DAC) using SSMS?
This error occurs when you try to connect to DAC using the right click Connect and the  Connect to Server dialog from the Object explorer 

However, you can connect to DAC via SSMS by clicking the Database Engine Query button 



The Connect To Database Engine dialog will pop out. Type the "admin:" prefix before the server name and you'll be connected to DAC



Hope this helps