As soon as the campaign is concluded and the funds from Kickstarter has been received (if the campaign is successful!) the Pledge Manager will be released shortly afterwards.
As long as you have backed the Campaign at any of the pledge levels you will be granted access to the Pledge Manager.
Like most things at WDM, we've already been doing what we can behind the scenes and the Pledge Manager is one of them as it has already been worked on. We will be using BackerKit as per our previously successful Kickstarter so you will be able to add additional items of any unlocked stretchgoals.
Don't see the answer to your question? Ask the project creator directly.