I think you are using the windows build of mojoportal. You need to use the "for-mono" release files (see the list of files on the download page at Codeplex) . The Mono build includes mojoPortal.Web.MonoCompilationSection, the windows build does not.
Or, if you have MonoDevelop you can use the source code and build the mojoportal.mds solution.
Hope it helps,
Joe