What does "auto-mount" do in VirtualBox shared folder setup?

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,

http://www.virtualbox.org/manual/ch04.html#sf_mount_auto

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.