I was googling unsuccessfully.
I've created Aura components without any problem, but I can't figure out how enabled this:
I'm trying to enable that feature in a Dev Winter 15.
To be very precise ,You will need to request salesforce to provide you the access to this builder .since its in pilot you will need to raise a case to get early access to this for your instance.
You can follow this link to open a new developer edition.
The Lightning App Builder is a new declarative tool for Admins to use to build pages that customize the behavior of Salesforce1. This feature is in pilot for Winter '15.