Could it be that the ConfigSections should be added to your web.config before the </system.webserver> part?
Here's an example that worked:
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<configSections>
<section name="managedFusion.rewriter" type="ManagedFusion.Rewriter.Configuration.ManagedFusionRewriterSectionGroup"/>
</configSections>
<managedFusion.rewriter xmlns="http://managedfusion.com/xsd/managedFusion/rewriter" />
<location path="xas">
<system.webServer>
<httpErrors errorMode="Detailed" />
</system.webServer>
</location>
<system.webServer>
<validation validateIntegratedModeConfiguration="false"/>
<modules runAllManagedModulesForAllRequests="true">
<add name="RewriterModule" type="ManagedFusion.Rewriter.RewriterModule, ManagedFusion.Rewriter"/>
</modules>
<handlers>
<add name="RewriterProxyHandler" preCondition="integratedMode" verb="*" path="RewriterProxy.axd" type="System.Web.HttpForbiddenHandler, System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"/>
</handlers>
<staticContent>
<mimeMap fileExtension=".mxf" mimeType="text/xml" />
</staticContent>
</system.webServer>
</configuration>