Have you upgraded umbraco to version 2.4 after you installed the rss extension?
Please confirm that this XML is found in the file:
/umbraco/config/create/UI.xml
<nodeType alias="initrss">
<header>RSS Feed</header>
<usercontrol>/create/simple.ascx</usercontrol>
<tasks>
<create assembly="umbracoRss" type="BusinessLogic.rssCreateTasks"/>
</tasks>
</nodeType>
<nodeType alias="rssInstance">
<header>RSS Feed</header>
<usercontrol>/create/simple.ascx</usercontrol>
<tasks>
<delete assembly="umbracoRss" type="BusinessLogic.rssCreateTasks"/>
</tasks>
</nodeType>
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