There are two workarounds available that might help you.
(Unfortunately Microsoft decided not to support iCal-streams in Outlook for Mac. This is true for both Outlook 2011 and Outlook 2014.)
Workaround 1: Setting up the subscription through Outlook Web App
If you are using Office 365 or have the Outlook Web App enabled in your Exchange server (2010 or newer) you can use your webmail to set up the subscription. This subscription will then be transfered to your Outlook-client.
- Login in to your webmail. This is found here if you use Office 365. If you have an own Exchange server your webmail can usually be found at http://mail.yourcompanyname.com or http://www.yourcompanyname.com/mail. Please contact your IT-support if you can locate your Outlook Web App
- Set up a calendar subscription in your Outlook Web App according to these instructions for Office 365 or Exchange 2013 and these instructions for Exchange 2010
- Restart your Outlook client on your Mac
Workaround 2: Using Apple Mail, Calendar and Contacts as a replacement
Your Mac is ready out of the box to work with Exchange (the underlying technology of Microsofts Mail, Calendar and Contact platform). The built in programs Mail, Calendar and Contacts is a full replacement of Outlook with very few features missing. The built in programs tend to be a bit more light weight, faster and less power hungry.
To set up your Mac you simply go into "System preferences > Internet accounts > Exchange" and follow the instructions. If you are missing a piece of information, please contact your IT-support. The information needed is the same as when you connect your phone to your email and calendar.
You are all done and can continue booking those meetings and sending those emails with Apples built in programs.