Unlike Windows, in a Linux guest you don't access the folder as if it were a drive or network share but you mount it to part of the file system tree as it's own type of filesystem.
Assuming you have virtualbox guest additions running on your Debian guest, run the following on the command line (substituting the Folder Name from the "Edit Share" dialog and omitting the brackets):
sudo mount -t vboxsf [Folder Name] /mnt/newfoldername
This will use the superuser account (aka "root") to mount the shared folder (of type "vboxsf") to the /mnt dirctory with access limited to the "root" account.
Try sudo ls /mnt (using the superuser account to list the contents of /mnt) to check if it worked. If it lists the contents of the directory then it's working and you can move on to other mount methods (e.g.: editing /etc/fstab) that will allow access to other accounts
Symlink for the shared folder from the guest machine is disabled by default, you can enable it with the following command from the host. My OS on the host computer is windows, so it would be like:
C:\Program Files\Oracle\VirtualBox\VBoxManage.exe setextradata VM_NAME VBoxInternal2/SharedFoldersEnableSymlinksCreate/SHARED_FOLDER_NAME 1
Where 'C:\Program Files\Oracle\VirtualBox' is the installation location of VirtualBox, 'VM_NAME' is my virtual machine name, and 'SHARED_FOLDER_NAME' is the name of the shared folder in the host machine.
If you are using Linux on your host, you can use the same command as below:
VBoxManage setextradata VM_NAME VBoxInternal2/SharedFoldersEnableSymlinksCreate/SHARED_FOLDER_NAME 1
Remember it is required to shut down all running virtual machines and close the VirtualBox before execution of the commands. After that windows users run it as an administrator privilege (right click on Virtualbox and run as admin). Now you can create a symlink in a regular way:
ln -sv /to/source/directory /to/destination/directory