Unity needs to get the SID of a user in the domain that maps to a subscriber in it's database such we can authenticate the user and see if they have rights to attach to the admin console.
You can configure IE to prompt for a login/PW for all connections and then provide the login/PW of a user's domain account that has admin rights in Unity if you want to do it from out of the domain like that. Either way, when you hit the SA page Unity will need a SID of an account that maps to a user. You can use the GrantUnityAccess tool to create such a mapping (see the help for the tool in Tools Depot for more details on using it).
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.