How do I make Ubuntu Linux full screen in Oracle Virtual Machine

I have installed Ubuntu in a virtual machine by following this article. I called the VM "myLinux", I succeeded to install it, but linux is not in full screen.

I made some searches and tried to fix it by Insert Guest Editions CD image, but it gave me an error:

Unable to insert the virtual optical disk C:\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso into the machine myLinux2.
Could not mount the media/drive 'C:\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso' (VERR_PDM_MEDIA_LOCKED).

Result Code: E_FAIL (0x80004005)
Component: ConsoleWrap
Interface: IConsole {872da645-4a9b-1727-bee2-5585105b9eed}
Callee: IMachine {5047460a-265d-4538-b23e-ddba5fb84976}

I tried Remove disk from virtual drive from the Optical Drives menu, but it didn't help.
Please help, it's really frustrating.