Easy work-around for "can't choose category in design center"
I was having the problem where you can't click a category on the pop-up box because it would disappear too quickly, and discovered an easy work-around: narrow the size of your browser window. Apparently the site does some math based on the window width, so if you're using a high resolution display, the pop-up is placed too far away from the original box and you can't get to it. Using a "skinny" window seems to do the trick.
Hope this helps!