Tinkercad Beta is accessible for anyone with a Tinkercad account through their Dashboard at www.tinkercad.com.
Just click "Create new design with Beta".
Note that any Tinkercad Beta designs you create will show a blue "Beta" tag on the bottom of the thumbnail. Clicking "Tinker this" will open that design in Tinkercad Beta editor.
Otherwise "Create new design" will create a new design in the regular old Tinkercad!