Dedicated Administrator Connection (DAC) is a new feature of SQL Server 2005; it is a special diagnostic connection for administrators that can be used to connect to a server when standard connections are not possible.
If you are trying to connect to a server using the DAC with SQL Server Management Studio and you keep getting the following error:
Dedicated administrator connections are not supported. (ObjectExplorer)
it means that you are trying to connect Object Explorer using the DAC.
Object Explorer cannot connect using the DAC; only Query Window can. That means that you cannot press the New Query button; you have to use the Database Engine Query button.