I am no expert at importing plug-ins.
But you can install the plugin in 2 ways:
1. within calibre->preferences->plugins->get new plugins .
2. download the zip file in this thread, and use:
calibre_>preferences->plugins->load plugin from file.
I don't know which way you took?
You could try the other, to be sure.
(the file you used could be damaged, or renamed).
Till now there were no issues reported like this, dispite it has been download over 300 times.
I see you are using linux? which version of calibre ?