To access Report Builder, sign in to ArcGIS Online and click on InstantAtlas Report Builder in the app launcher available in the top menu.

 

Alternatively, you can browse directly to https://reports.instantatlas.com and sign in using your ArcGIS Online named user.

 

If you are unable to sign in, it may be that the administrator of your ArcGIS Online organisation has not assigned a Report Builder license to your user.