I leave this post here, perhaps it helps others:
The plugin requires Zend:
/**
* Usage: http://mail4us.net/myroundcube/
*
* Requirements:
* Get Zend GData APIs http://framework.zend.com/download/webservices
* Copy and paste "Zend" folder into ./program/lib
* -> File structure must be: lib
* - Zend
* - Loader.php
* - ...
*
* NOTICE: Patch ./program/lib/MDB2.php
* http://pear.php.net/bugs/bug.php?id=17039&edit=12&patch=skip_delimited_strings_fix_quoting_array&revision=1264618739
* Fixed since Roundcube SVN Trunk 3354 (http://trac.roundcube.net/ticket/1486547)
*
**/