can't remove DRM with Calibre on Ubuntu 12.04

[rawimage]

hello,

newcomer: sorry if I talk non-sense. I'm also new to the eReaders' world...well, here it goes:

1) I don't have a Kindle device. I have a Sony (PRS-650) which I connect to Ubuntu 12.04 via Calibre 0.8.38. I have Kindle-for-PC 1.10.4 installed on Ubuntu (under Wine 1.5.6);
2) following Alf Apprentice's instructions (hopefully from the right page) I installed 5 plugins to Calibre (tools_v_5.4.1 from folder "Calibre_Plugins");
3) I didn't customize any of the plugins (I don't have a Kindle device);
4) I bought an eBook locked by DRM from Amazon. I can import it to Calibre no problem. When I click on convert, I get a message:

"This book is DRMed. Cannot convert...etc".

Can you please help? I must have missed out something from AA's instructions, but don't know what exactly...thanks!

[rawimage]

quick update: I tried to remove the DRMed book from the Calibre library, uninstall + install Kindle-for-PC, import this book once again into Calibre. It does import it, although the following message pops up:

Error: No key found in 0 keys tried. Read the FAQs at Alf’s blog. Only if none apply, report this failure for help…. /home/daniele/My Kindle Content/B004FV4T8O_EBOK.azw

(if I click on OK the book gets imported)…weird?

I also tried to upgrade Calibre to the latest version for Linux, to no avail...any idea?