I believe the default is Symantec unless it was changed when setup. try it, if it does not work you would have to contact Symantec to have their tech support either remote in to reset it or show you how to do it.
To reset the Symantec System Center admin user password
On the computer running Symantec System Center, start Windows Explorer.
Go to \Program Files\Symantec\Symantec System Center\Tools.
In the right pane, double-click the IFORGOT.exe file.
In the Primary server field, type the name of the server group’s primary server.
In the user field, type admin
In the New Password and Confirm New Password fields, type the new password.
Click Reset Password.
You may be prompted for a Windows user name and password if you specify a remote server.