How to get the GRUB using virtualbox?

You have to know that VirtualBox is just Virtual, so many things can be different.

Reboot your virtual machine and when you see Oracle's logo , click the screen (focus there) and then hold down Shift until Grub menu appears.

Edit /etc/default/grub:

gksudo gedit /etc/default/grub

And add a # in front of GRUB_HIDDEN_TIMEOUT=0

Finally, apply with:

sudo update-grub

Credits: https://askubuntu.com/a/16049/263353

Note: If you plan to change GRUB2 shell keyboard layout, forget immediately about using the Shift method. It may be normal that terminal_input at_keyboard make this key not work anymore.