In the admin dashboard - you have the “Cornerstone” menu item hovering the menu gives you.
There should be an option “Open Cornerstone” or similar.
Then if you click “Theme Options” the builder opens but the theme options panel is hidden. This should open with the Theme Options panel open.
Currently we’re confronted with a blank preview screen and the document stating “No active document”.
Think Kory mentioned that this would be resolved in upcoming beta. a few people mentioned there was no way from the dashboard to open cornerstone. I believe the cornerstone menu item in the dashboard will open cornerstone directly in the next release
Think I’ve maybe missed a few posts then… I probably need to get back to work any way 
I have the fortune of being on holiday in the Netherlands visiting family… so I have a little more free time than normal
@urchindesign, as @scotbaston mentioned this has been addressed and we will make sure to tidy up some of these loose UX ends. Thank you for letting us know though!
@scotbaston, I wish I had the fortune of being on holiday in the Netherlands…maybe soon…