There are two places to allow for pre-release updates -- you can right click on the task tray, choose settings, go the general tab, and then click the box to allow pre-release updates. Unfortunately, this box doesn't do anything.
You have to click the impulse icon on the top left corner of the impulse reactor window, and use THAT to allow pre-release updates. This one works.
The one on the task-tray needs to be fixed.