Hi Siraj,
            I am no expert :-) but at first glance I can see the following issues
1) the cmp-connection-factory entry is not required for a session bean. Also the syntax is wrong.
It should be
<cmp-connection-factory>
                <resource-link>
DefaultDatasource</resource-link>
</cmp-connection-factory>
instead of
<cmp-connection-factory>DefaultDatasource</cmp-connection-factory>

2)Another thing that is not clear is if 
<alt-dd>j2ee-deployer-plan.xml</alt-dd> is your openejb-jar.xml file
<module>

        <ejb>helloworld-ejb.jar</ejb>

        <alt-dd>j2ee-deployer-plan.xml</alt-dd>

    </module>

In the alt-dd element you need to give the geronimo specific deployment descriptor of the
ejb you are trying to deploy i.e. the equivalent of the openejb-jar.xml.

3) A NullPointerException is not at all supposed to occur even if your plans have errors. So you may want to open a JIRA issue.

Hope this helps

Regards
Manu