In VirtualBox, Ubuntu guest, Windows 7 host, I click this "Auto-mount" box, but it doesn't seem to have any effect. I have to edit
fstab to get it to mount. So what is it for?
Make sure that the additions are installed, and that you have added your username as a member of the vboxsf group (
sudo usermod -aG vboxsf <your username>).
Then you need at least to log out (or reboot) for the change to take effect.
In Ubuntu this should auto-mount shared folders under
/media/sf_share_name. I've had permissions issues with the auto-mounts and still use the fstab.
You can find information on that in the VirtualBox Manual,
Maybe your user is not in the vboxsf user group, maybe you didn't look correctly where to find it. Are you sure you installed the latest guest additions?
You can check all that, but you can refer to the manual for details if it helps.