I always run into a problem attaching a shared folder to my Ubuntu virtual box machine. This is what works for me.
1. Make sure you have the latest version of guest additions installed.
2. Add a shared folder using the virtual box menu and auto mount it
3. Turn on the virtual machine. Browse to /media/sf_*sharedfoldername*
4. By default with auto-mount, the shared folder will have permissions assigned to vboxsf. You need to add your current user and maybe also root to the vboxsf group so they get access to the shared folder
5. usermod -aG vboxsf <youruser>
usermod -aG vboxsf root
6. Restart virtual machine
7. Now you should have access to the shared folder under /media/sf_*sharedfoldername*