how to load classpath entries for an SDK?

I can see in the Expression Evaluator window that the Gosu JAR entries are loaded (see screenshot "idea_expression_evaluator.png").
how can I access them? and/or why aren't they shown on the SDK's "classpath" tab?

UPDATE: one thing I noticed is that Gosu Jars URLs start with "file://", while Java ones start  with "file://" - that might be the reason...

btw, this is the code I'm using to set up the SDK:

  public static void setUpSdk(Sdk gosuSdk, @Nullable Sdk javaSdk, Sdk[] allSdks) {
    GosuSdkAdditionalData data = new GosuSdkAdditionalData(gosuSdk, javaSdk);

    String sdkName = GosuSdkType.SDK_NAME;
    sdkName = SdkConfigurationUtil.createUniqueSdkName(sdkName, Arrays.asList(allSdks));

    final SdkModificator sdkModificator = gosuSdk.getSdkModificator();

    sdkModificator.setName(sdkName);

    if (javaSdk != null) {
      sdkModificator.setVersionString(javaSdk.getVersionString());
    }
    sdkModificator.setSdkAdditionalData(data);

    addJavaJarsToOurClasspath(javaSdk, sdkModificator);
    addOurJarsToClasspath(sdkModificator, gosuSdk);
    sdkModificator.commitChanges();
  }

  private static void addOurJarsToClasspath(SdkModificator sdkModificator, Sdk gosuSdk) {
    File gosuRootDir = new File(gosuSdk.getHomePath());
    File gosuJarsDir = new File(gosuRootDir, GOSU_SDK_JARS_FOLDER_NAME);
    File[] jars = getFilesInDirectory(gosuJarsDir, FILE_EXTENSION_TO_ADD_TO_GOSU_SDK_CLASSPATH, false);
    for (File file : jars) {
      String urlString = VfsUtil.pathToUrl(file.getAbsolutePath());
      VirtualFile virtualFile = VirtualFileManager.getInstance().findFileByUrl(urlString);
      sdkModificator.addRoot(virtualFile, OrderRootType.CLASSES);
    }
  }

  private static void addJavaJarsToOurClasspath(Sdk javaSdk, SdkModificator sdkModificator) {
    for (VirtualFile file : javaSdk.getRootProvider().getFiles(OrderRootType.CLASSES)) {
      sdkModificator.addRoot(file, OrderRootType.CLASSES);
    }
  }

  public static File[] getFilesInDirectory(File dir, String suffix, boolean recursive) {
    String[] dirContents = dir.list();
    List result = new ArrayList();
    for (int i = 0; dirContents != null && i < dirContents.length; i++) {
      String dirContent = dirContents[i];
      if (dirContent.endsWith(suffix)) {
        File file = new File(dir, dirContent);
        result.add(file);
        if (file.isDirectory() && recursive) {
          result.addAll(Arrays.asList(getFilesInDirectory(file, suffix, recursive)));
        }
      }
    }
    return (File[]) result.toArray(new File[result.size()]);
  }

ok, I figured it out.

I should have added JAR entries instead of regular files:

      VirtualFile virtualFile = VirtualFileManager.getInstance().findFileByUrl(urlString);
--> NEW -->        VirtualFile jarFile = JarFileSystemImpl.getInstance().getJarRootForLocalFile(virtualFile);
        sdkModificator.addRoot(jarFile, OrderRootType.CLASSES);