Check your xml descritor files

Ohhh now i understand why everybody is talking about the helloworldportlet, sorry i didn't know we had such an example in the doc.

There is an error in the doc.
In helloworld-pages.xml:

replace:

<instance-ref>/portal-hello.HelloWorldPortlet.HelloWorldPortletInstance</instance-ref>

<instance-ref>/helloworld.HelloWorldPortlet.HelloWorldPortletInstance</instance-ref>

the context name doesn't seem to make the difference.
I checked it over several times, tried to deploy as an EAR and as a WAR with no success. Looks like something's missing in the doc. Perhaps a web.xml entry (Servlet, listener)? I started to debug the issue, but haven't figured it out yet (too much new code and my head's spinning ;)

user error: my war didn't contain a portlet.xml ; my ant template didn't add it in; once the correct context name is used it works (thanks Thomas!). I run into other problems now, but I assume for the moment that those are user errors as well..... boy, do I feel stupid :(

Had the same problem with the example file downloaded on the site.
Added a / in the helloworld-pages.xml

<pages>
 <portal-name>default</portal-name>
 <page>
 <page-name>helloworld</page-name>
 <window>
 <window-name>HelloWorldPortletWindow</window-name>
 <instance-ref>/helloworld.HelloWorldPortlet.HelloWorldPortletInstance</instance-ref>
 <default>true</default>
 <region>left</region>
 <height>0</height>
 </window>
 </page>
</pages>