Moving this discussion to the Flash Builder. I don't believe you will be able to downgrade to 4.5 as part of your Creative Cloud subscription. Do you receive a specific error message when you try to use Design mode?
No error, just a blank white area. When I drag things to it I get a NO sign like the Ghostbusters sign, forgot what that's called.
When I breifly had 4.5, I could easily drag things over and create sites. Then it automatically upgraded and suddenly, doing the exact same things, stopped working.
I'm still learning FB so I can't convert to pure code yet. If I can't get the design mode to work then I'll be dumping FB since I can't follow any of the tutorials.