After pressing the "new flow" button I get a screen that has title "unsaved flow" and it's blank underneath. I've tested on firefox and IE. My flash is up-to-date. I've tried clearing my browser's cache.
I experienced the same. This is related to changed security policies in Chrome.
To resolve this I had to add my salesforce domain as an exception in Chromes Flash content settings:
In a new tab, go to chrome://settings/content/flash and add my salesforce domain to the allowed exceptions
If you use a different browser, you may have to install the Flash Plug-in installed to use the Flow designer.
PS: It might be also helpful to clear the browser cache.
In a new tab, go to chrome://settings/clearBrowserData
On the Advanced tab
Select data from "All time", Check "Cached images and files.", click Clear Data at the bottom.