When you open Manager, you will be prompted to log in. The required account privileges depend on which software and platform you are using:
The account also needs to be a member of the agsusers group on any machines to which you will make an ArcGIS Server Local connection.
.NET only: You will notice that the name of the ArcGIS server machine cannot be changed in the login screen. This is because Manager is directly associated with one instance of ArcGIS Server. When you run the Web Applications Post Install, you specify the instance name and the GIS server associated with that instance. You can only use Manager to log into that one GIS server. Specifically, the GIS server is the machine running the server object manager (SOM) component of ArcGIS Server.
When you log in to Manager with the default shortcut, the name and password are sent to the server in clear text. For working with Manager outside of your local machine, it's recommended that you run Manager using Secure Sockets Layer (SSL) which encrypts all communication between Manager and the server. See Setting up SSL for an example of how to set up SSL using the IIS web server.
If you run Manager under SSL, be sure to change the Manager shortcut to use https instead of http. To do this, right-click the shortcut, click Properties, and modify the target URL so that it begins with "https".