I'm teaching CS5 (not 5.5) using networked computers. The media query button in the multiscreen preview is always greyed out. I've found through these forums that this is because the file is located over a network and thus doesn't have a direct URL (i.e. C:/mypage.htm) but a network location (i.e. ///mypage.htm).
Does anyone know a workaround to this bug or has it been patched? Is this bug fixed in CS5.5 and CS6? It's kind of a pain to have to code the CSS3 media queries by hand.
Had the same problem, but it's a simple solution.
Make sure that you do not have your CSS selected in the tiny menu bar. Select your "Source Code" and then your multiscreen preview will always show up.