From your app directory use npm install mojito-session-addon to install the addon.
This is a simple session management library that uses Y.Cache underneath.
Add "mojito-session-addon" to the requires:[] array of a mojit controller and the session will be available in following fashion.
In the controller use as actionContext.session.get("key"); and actionContext.session.set("key","value");.
For example check the gist: https://gist.github.com/akshar100/5763632