I tried to re-download the plugin from the Arduino web editor by clicking the yellow bar at the top that notified me that notified me that no plugin was detected. After I clicked and the new troubleshooting window popped up, I clicked download again, but nothing happened.
When I hover over the 'download again' in that pop-up, the link address that my browser shows is: create-staging.arduino.cc/ide/#/getting-started/2 . Entering that link directly in my browser takes me to a static(?) "Welcome Back" page.
I was able to download the plugin again from the "Getting Started" walkthrough.
My system info: Mac 10.10.4 and Chrome 47.0.2526.80