To my surprise, everything went fine with installation of latest Duokan 2012. So, for Kindle Keyboard users, firmware 3.4. here's what to do:
1. Copy DK_System folder on root of Kindle
2. Copy three bin files from the Kindle 3.2.1 folder (for kindle 3.3) on root
3. Disconnect Kindle from PC
4. Menu/Settings/Update your Kindle
5. Be patient. It will update eventually.
Now, you will have a dual boot screen, choose Q for Duokan or W for Kindle.
If someone has User Manual in English, it would be great, as this is the furthest I can go. Don't have a clue what to do next, to import pdf books and try to change the language settings (although I believe it would be impossible).
Thanks Thomass for your replies!