I have seen some people want to make shared folder between VirtualBox and ubuntu but it is sometimes tricky for them to make a shared folder:
1- First install “Insert guest Additional CD” from Devices. In ubuntu VM, you may install those sh files first.
2- restart the system
3- Make a Shared Folder from Settings> Shared Folder
4- Add your ubuntu user to vboxsf group
sudo adduser $USER vboxsf