Check for parent/mod as well as parent.mod in findext. Debian/Ubuntu ship a system-wide hgrc that loads extensions using the syntax hgext/extension=, rather than hgext.extension=. Although the extensions work correctly, hg help <extension> doesn't understand this form without this patch.

<item>
    <title>#tag|escape#</title>
    <link>#url#?cs=#node|short#</link>
    <description><![CDATA[#tag|strip|escape|addbreaks#]]></description>
    <pubDate>#date|rfc822date#</pubDate>
</item>