On the Parse JS changelog:
Removed the concept of the concept of the current user when running in node.js Current users can be enabled in node with Parse.User.enableUnsafeCurrentUser() Many requests now support passing an explicit session token as an option
Unfortunately none of their documentation has been updated to support this.
You'll have to dig into their source code to see how { sessionToken } is being used.
(Basically, in most calls to Parse, you can add sessionToken: <token> to the backbone options)
To allow server side login as before, just paste this line somewhere before the login
Parse.User.enableUnsafeCurrentUser();
Isen