Did you install JOSSO gateway and agent? If location http://localhost:8080/josso/signon/login.do is not available, then it seems that you forgot to install josso gateway to your server.
By the way, which version of reference guide are you reading? Reference guide for JBoss Portal 2.7.1 from standard documentation page http://jboss.org/jbossportal/docs/index.html is related to older version of josso. Configuration with josso 1.8.0 is little different.
You can take a look to the SSO frameworks integration guide with JBoss Portal, which is at http://docs.jboss.org/jbportal/sso/ . In chapter 2 is detailed tutorial for integration of JBoss Portal 2.7.2 with josso 1.8.
Hope this helps.