The current versions of
FCKeditor (around 2.2) contain
a bug that prevents the Quick File Uploader to work by default.
The solution is to search FCKconfig.js for "FCKeditor.QuickUploadLanguage" and replace it to "_QuickUploadLanguage".
If you ever need to debug FCKeditors file upload functionality, remove
target="UploadWindow" from editor/dialog/fck_image.html to get some feedback with uploads.