I'm not sure how the Kolab calender will work with a standard install of RC 8.1.
I think most people use the plugin manager and download the calendar from Rosali and use that with RC.
You can get the plugin manager plugin from
here, once you have it you need to register the plugin in your main.inc.php, in the below section.
You will need to place the "plugin_manager" folder in your plugins directory. There are also some other items needed for the plugin manager to work, but the site has good instructions on how to get it all working. I suggest that you read through as once you have it set up it is very useful.
/ List of active plugins (in plugins/ directory)
$rcmail_config['plugins'] = array('global_config','plugin_manager',....);
Once the plugin is active, and you have added your user in as the admin as per instructions you will be able to download the latest plugins and can upload these to your plugins directory.
I suggest just using a few to start off with then add others one by one and make sure they work for you. It is much easier than loading a whole lot of new ones and finding, which one is a problem if you have any.