can't remove DRM with Calibre on Ubuntu 12.04

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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!