The URL which you have mentioned in your post is not a Portal URL. It is the URL for the Enterprise Manager Console. Your Enterprise Manager Console may not be up and running. You can use the emctl command to start it :
emctl start iasconsole
Your Portal URL will look like http://localhost:port/pls/portal (where port represents the port number at install, 7777 by default on Unix systems unless infrastructure is running on this server as well and 80 by default on Windows systems). Check the portlist.ini in your $ORACLE_HOME/install directory for the port numbers in use by your installation.