Bug 29755 Attach source does not allow one to select a folder... [build path]
When opening some .class that does not yet have a source attachment eclipse shows a text version of the interface defintion and a button for Attaching source. When pressing this button a window appaer where one can enter the name/location of a folder, zip or jar for the source. There are also a button for choosing it via a file selection dialog and this works fine for zip and jar files - but not for selecting a directory :( Workaround is just to enter the directory manually.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/SourceAttachmentBlock.java
found= true;
			} else {
				newEntries.add(curr);
			}
		}
		if (!found) {
			if (newEntry.getSourceAttachmentPath() == null || !putJarOnClasspathDialog(shell)) {
				return;
			}
			
			newEntries.add(newEntry);			
		}
		IClasspathEntry[] newClasspath= (IClasspathEntry[]) newEntries.toArray(new IClasspathEntry[newEntries.size()]);
		jproject.setRawClasspath(newClasspath, monitor);
	}
	
	private boolean putJarOnClasspathDialog(Shell shell) {
		final boolean[] result= new boolean[1];
		shell.getDisplay().syncExec(new Runnable() {
			public void run() {
				String title= NewWizardMessages.getString("SourceAttachmentBlock.putoncpdialog.title"); 
				String message= NewWizardMessages.getString("SourceAttachmentBlock.putoncpdialog.message"); 
				result[0]= MessageDialog.openQuestion(JavaPlugin.getActiveWorkbenchShell(), title, message);
			}
		});
		return result[0];
	}
	
	
}