At virtualbox, install Guest Addition. Click Devices -> Install Guest Additions…
At Ubuntu, click on Places (on top bar, left) and click Vbox Guest Addition. Install it (using autorun).
Restart Ubuntu (guest OS).
Add Shared Folder. Click at Devices -> Shared Folder -> Add Shared Folder. Set it to permanent and auto mount.
Add your account (in Ubuntu) into Virtualbox special Group.
- Click System -> Administration -> Users and Group.
- At Users Setting, click at Manage Groups
- at Groups Setting, scroll down until you see vboxsf
- Click at vboxsf and click button Properties.
- Add your account into this group by click on your name (under Group Members).
- Restart Ubuntu.
Now you can access your shared folder in /media/your_shared_folder_name