The zip package of CMandrill will install both the extension and the plugin. After the installation you need to navigate to components -> CMandrill and then access the options of the extension. Once you provide your Mandrill API key, you need to navigate to Extensions -> Plugin manager and activate the "System - Mandrill" plugin.
To access the options screen navigate to Components -> CMandrill and then click on the Options tag.
In the basics tab you have the following parameters:
API key - In the API key field provide your Mandrill API key (on the day of writing that documentation the API key could be found by clicking on the settings button (top right) at the mandrillapp.com page (you need to be logged in to see it))
Use https - The use https option defines if the plugin should communicate with Mandrill server through an SSL connection or not. We recommend to set this option to yes if your hosting has the necessary PHP SSL libraries for this.
In the Permissions tab you can specify which user group is allowed to access the backend of the extension.