I have installed a fresh copy of Ubuntu Server 12.04 and VirtualBox 4.3. I have set up a VM called "MediaServer" and tried to start it. I then get the following error:

Cannot open host device '/dev/sr0' for readonly access. Check the permissions of that device ('/bin/ls -l /dev/sr0'): Most probably you need to be member of the device group. Make sure that you logout/login after changing the group settings of the current user (VERR_ACCESS_DENIED)

I have looked all over the Internet and have been unable to find a solution. Using Webmin, I tried changing the group settings so that my user name was in the "vboxusers" group, but that did not work either. I tried various other changes in group settings and none of them worked.

Also, I tried rebooting the server after the changes and that didn't work either.

I have been following a guide on how to set up an Ubuntu server from the website "linuxhomeserverguide.com" and when it came to the section where you could finally set up your first virtual machine, I am stumped. I would really appreciate it if someone could help me. Thanks in advance.