You access the free trial of Report Builder for ArcGIS via the ArcGIS Marketplace.
Browse to the listing in Marketplace and sign in with an ArcGIS Online user with the Administrator role.
Click the 30-Days Trial button. No payment is required.
Fill in the form, read the T&Cs and submit your request.
You will receive an email from Marketplace that contains useful information to get started. It will tell you that you need to assign license(s). Sign in to Marketplace and click Console.
Under Apps & Data, click the Manage Licenses option against Report Builder for ArcGIS.
Click the Manage option for Report Builder for ArcGIS.
Assign the license(s) to user(s) using the toggle buttons. The number of available licenses may vary.
Once this as been done, the user will be able to access Report Builder via the Quick Launch option when they sign into ArcGIS Online.