I'll start saying thanks to Roland for helping me with the calendar pluggin.
1. First, install webcalendar from the original site.
After this, configure the calendar pluggin. This is what Roland recommends me and i also did:
2. In the file: ./myplugins/collection/calendar/config.inc you need to leave:
$rcmail_config['calendar_url'] = "./myplugins/collection/calendar/app/";
Even if you did install the webcalendar in another place. I also recommend install it in another place just in case you want to use the calendar without roundcube, but always doing reference to the same database (if you want).
3. Then in the file: ./myplugins/collection/calendar/app/includes/settings.php
You will need to put the information of your database connection, the same information when you did the default installation of your webcalendar. This file contains this structure:
/* updated via install/index.php on Fri, 20 Feb 2009 17:48:59 +0100
install_password: 7c7d2949bfce237294aa0040cb4ecdcf
db_type: mysql
db_host: youdomain.com or localhost
db_database: webcalendar_databasename
db_login: root (or your user name)
db_password: yourpassword
db_persistent: false
readonly: false
user_inc: user-imap.php
use_http_auth: false
single_user: false
mode: prod
# end settings.php */