JBoss Portal will only work on JBoss AS 4.
There was some initial effort to get it working on JBoss AS 5 (http://blog.jboss-portal.org/2009/03/jboss-portal-running-on-jboss-as5.html, http://community.jboss.org/wiki/JBossPortalonAS5) but this was only a technology preview and is no longer being actively worked on.
The good news is that JBoss Portal and eXo are working together on a portal project called GateIn (see www.gatein.org). This project will take the best of the technologies from JBoss Portal and eXo Portal and incorporate them into a single portal project. This will be the logical successor of JBoss Portal and eXo Portal.
GateIn is currently still in Beta, but it does work on JBoss AS 5.1 (and Tomcat).