We just got a postcard today from Moz saying the following:
You may have heard that custom reports aren't available in our new software platform, Moz Analytics, yet. We're working on making them easier to use, with more flexible reporting options--and more beautiful, with cool visualizations of your data.
In the meantime, custom reports will be hanging out in PRO until they're available--and awesome--in Moz Analytics.
We know it's inconvenient not to have this feature available in your Moz Analytics workflow, and we're here for you. Just give us a shout on Twitter @Moz, and we're happy to chat!
So, it seems that you have to click the "Back to PRO" button to use your custom reports, for now.