Hi Prabhu,

Use ADMIN:servernameinstancename

For Example:
if the server name is servertest and the instance name is SQL1
then use

ADMIN:servertestSQL1

Please note:
1. Dedicated Administrator Connection is only allowed from a client running on the server. By default network connections are not supported.

2. Only users with SQL Server sysadmin role can use DAC.

Please refer to below link for additional details.
http://msdn.microsoft.com/en-us/library/ms189595(v=sql.105).aspx

Thanks,
Devi Prasad.

I tried the above couldn’t succedded, resulted in error stating that server not found or unavailable.

please suggest.

Thanks,
Prabhu