The question mark is indeed an idiom for indicating if a property is required or not. It is not to mark if the property can have or cannot have a value. That's done using
Did you install the API Workbench via APM or the code base? The team is currently working on the support for RC2/GA and will be able to push the final version to APM. Currently, you can basically install the RC2/GA version from its sources here.
I am using that version, and defining