@miriamele It might be a problem on your side, I just installed the plugin on my XUbuntu 14.04.3, with Calibre 2.58
Though with regard to libimobiledevice, you might have to manually install using apt-get. Because I seem to notice that I have libimobiledevice already installed, and from their website, it seems to be a library for communicating with iOS devices.
To install it, use something like: sudo apt-get install libimobiledevice
Here's the package info on launchpad:
https://launchpad.net/ubuntu/+source/libimobiledevice