For landing pages and emails, you can do this in Design Studio by clicking on the landing pages or emails element in the tree, selecting the landing pages or emails in draft (holding down shift or control key), and right-clicking to approve.
You aren't able to do this for forms, which I assume is because Forms 1.0 didn't require approval to be made available on landing pages.
Just to clarify, even though you access this functionality from within the design studio, I recall all the marketing activities emails still show up there.
Notwithstanding it is a bit confusing and would be good to have the same functionality in the marketing activities side!