The Essentials team have blogged the step by step instructions on how to configure Microsoft Azure backup integration with an on-premises installation of Server 2012 R2 Essentials.
The blog post can be found here – http://blogs.technet.com/b/sbs/archive/2014/05/01/configuring-microsoft-azure-online-backup-on-windows-server-2012-r2-essentials.aspx
Note that there is currently an issue if you are using the Essentials role enabled on Windows Server 2012 R2 Datacenter edition in Azure (I have not verified if on-premises Datacenter edition is OK). You will receive an error, when attempting to install the Add-in – “The add-in cannot be installed because it is not compatible with WIndows Server 2012 R2 Essentials”. We hope that there will be a fix for this soon.