I am trying from some days to activate again the seamless mode on my VirtualBox(using Ubuntu 12.04 on an osx device), which was correctly set till now but which isn't working anymore. I have had already this problem but then I had to reinstall a new disk on my VB. I tried everything I know : installing it by clicking on the 'devices' item in the nav bar of VB's app, I created a new dir in /media and then tried to mount the /dev/sr0. When running sh ./VBoxLinuxAdditions.run, it installed correctly

But when rebooting my machine, I always have the same problem. Does anybody knows how to activate again Seamless Mode ?