not able to add buildin admin as login ..sql server 2008 express edition

i have added my AD group in the local administration of the server ... but facing issue while adding a new login .. (SA is disabled on the instance)

im getting below error 

Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\myuser>sqlcmd -S servername\instancename
1> CREATE LOGIN [BUILTIN\Administrators] FROM WINDOWS WITH DEFAULT_DATABASE=[mas
ter]
2> GO
Msg 15247, Level 16, State 1, Server servername\instancename, Line 1
User does not have permission to perform this action.
1>

referred :: http://sqlserver-help.com/2012/02/08/help-i-lost-sa-password-and-no-one-has-system-administrator-sysadmin-permission-what-should-i-do/

will putting the instance in single user mode .. make any difference to above method ?