Access scheme shows "Administrator" role only

What are the other roles that you have other than Administrator? Did you apply the permission that allows the different roles to be assigned to sections?

I have 8 other roles. What permission do you mean?

@marc.bau - Once you enabled the Workbench Access module, navigate to "admin/people/permissions/module/workbench_access" and see all the available permissions for the module. For the permission "Allow all members of this role to be assigned to Workbench Access sections", you need to select the user roles that you want to show up when configuring the Workbench Access access scheme.