The flash objects are placed in a way that they are not visible by default. The problem here is that when using a flash blocker, the replacement object is invisible, too. And as you have to right-click on the replacement object to enable flash for the site...
Alternative is to disable the flash blocker for the whole browser every time you want to use roll20. Not nice.
Even worse, GM interface doesn't even load without flash. It gets stuck on the black "Loading..." screen. Player screen worked on Sunday, just without jukebox and video conferencing.