Did you start with a clean installation of GateIn ? I think there are issues with extensions and a non-clean installation. If you're using JBoss and the embedded hsql DB, just delete the data directory of your AS installation and restart GateIn with the portal extension deployed. Also you can find documentation on portal extensions here http://docs.jboss.com/gatein/portal/3.1.0-FINAL/reference-guide/en-US/html_single/index.html#sect-Reference_Guide-Foundations-Extension_mechanism and there's also examples provided here https://anonsvn.jboss.org/repos/gatein/portal/trunk/examples/ where under extension is an extension to the portal container 'portal' and under portal is an example of creating a new portal contianer 'sample-portal'.
Thank you very much Nick,
Clearing out my embedded hsql DB did the trick.