>> took mine from the "StarDict Dictionaries -- 星际译王词库 词典下载" web page.
Yep, that's where i took mine too. Just tried again:
1. Removed existing 'dicts' folder on SD card. Boot Onyx, go to 'Dictionary' -> press the icon with book+arrows. It shows 2 dictionaries: 'English explanatory' and 'English-German'. Quit 'Dictionay'. Remove SD card. Put SD card into computer.
2. Downloaded stardict-longman-2.4.2.tar.bz2 from the above site. Create 'dicts' folder on SD card, unpack the archive there. The contents of SD card become:
3. Put the SD card into onyx. It opens file explorer @ 'SD Card' automatically. Shows new 'dicts' folder fine. Close file explorer ('back' button).
4. Click on 'Dictionary', then book+arrows icon: same 2 dictionaries shown, as in step 1). No 'longman'.