Once you shared a folder in VirtualBox with Ubuntu Guest, you need to install VirtualBox Guest additions, this can be done from menu.
Devices > Insert Guest Additions CD image
Once this is done, in your Guest OS, you will see a CD rom, install the provided software.
Before you can use shared folder, you need to add the user you use to group vboxsf, to do this, run following command in terminal.
sudo usermod -G vboxsf -a $USER
$USER is user name of the user you will be using. You can replace it with any user you will be using to access the shared folder.