Q: how do i get on photo stream

First you must meed these requirements on yhour Mac:

Minimum operating system requirements

You may also use iCloud with the following operating system versions:

iPhone, iPad, iPod touch

• iOS 5*

Mac

• OS X Lion v10.7.5 or later*

PC

• Microsoft Windows Vista (Service Pack 2)

* Note: Although you can use iOS 5 and OS X Lion as noted above, you will not be able to take advantage of some iCloud features, such as Shared Photo Streams, iCloud Tabs, Find My iPhone Lost Mode, Documents in the Cloud for iWork, and Find My Friends location-based alerts.

And you must be running iPhoto 9.4 or later. For Shared Photo streams you need OS X 10.8.2 or later.

Next enable Photo Stream in the System/iCloud preference pane

and in iPhoto's Photo Share preference pane.

That enables Photo Stream on your Mac.  You'll have to ask in the AppleTV forum rearding getting it to work.

Report back with your system and iPhoto versions that you are using.

OT