You could use XPATH, something like this, wrapped in a method:

    static string GetMetaDescription(HtmlDocument doc)
    {
        // get a META element recursively in the document
        // which has a NAME attribute equal to 'description'
        // and a non empty CONTENT attribute.
        HtmlNode node = doc.DocumentNode.SelectSingleNode("//meta[@name='description' and @content]");
        if (node == null)
            return null;

        // get the value of the CONTENT attribute
        return node.GetAttributeValue("content", null);
    }