This is achieved through either the main EM Home Page or through the Console.

Through the main EM website:

From http://<host>:1810, click on the "Application Servers Home Page Link" then select the specific instance you want to monitor.

Through the Console:

In the Navigator pane, click on "Application Servers", select the specific instance in mind, then click on it and then double click on the "Monitoring" button in the right pane explaining all the 9iAS related needed information. Oracle Enterprise Manager opens your default browser and displays the appropriate EM Home Page for the selected application server instance.