First, and foremost whilst there are three distinct form environments, you can only design a form in Development Environment.
Once you've clicked on the link in Form Details your form will load. As it is the Development Environment, you will see a login prompt and you must have the appropriate permission to access the form:
Login prompt
When logged in you need to go to a Designable page to start adding content.
Template pages e.g. the launch page, the guidance notes, declaration pages are not designable. Blank Who Are You and Blank pages are.
Once on a blank page click the 'Design tools' option on the top right. This will result in a slide out panel of functions appearing:
Design Tools - Before entering design mode
Click the 'Start Designer' option and the panel changes to show the eDesigner functions:
