If it helps, here is an example of one of my msbuild tasks...
<tasks>
<msbuild>
<executable>C:\WINDOWS\Microsoft.NET\Framework\v3.5\MSBuild.exe</executable>
<workingDirectory>D:\INETPUB\wwwroot\CruiseControl\Utilities\</workingDirectory>
<projectFile>Utilities.csproj</projectFile>
<timeout>900</timeout>
<logger>C:\Program Files\CruiseControl.NET\server\ThoughtWorks.CruiseControl.MsBuild.dll</logger>
</msbuild>
<artifactcleanup cleanUpMethod="KeepLastXBuilds" cleanUpValue="5" />
</tasks>
This should be nested under your project node.
Brandon Joyce