The thing is, dedicated administrator connection only works when there are no concurrent active connections. So, you gotta connect by clicking on "New Query" button - and then connect with
ADMIN:SERVERNAME
From SQLCMD:
sqlcmd -E -S servername -A
When a server level trigger prevents new connections to SQL server, then you might want to connect using the DAC and troubleshoot.
By default, only local dedicated admin connections are enabled - if you want to enable remote DAC, use Surface Area Configuration too.
exec sp_configure 'remote admin connections', 1
reconfigure