Check the InfoCenter for the detailed requirements for Portal 5.0.
http://publib.boulder.ibm.com/pvc/wp/500/ent/en/InfoCenter/wpf/inst_req.html The Enterprise Version of WAS includes PME, the base version does not. PME provides APIs that Portal requires.
I wonder if the reason you are not running is that you don't have PME, just base. I think base is always there, but for an enterprise install, PME is there as well.
I somehow got the idea that if you start the Portal install on a system with no WAS, it will install the necessary WAS for you, at least when installing from CD images.