After installing Ubuntu 16.0, when you want to install VBoxGuestAdditions.iso if you get the following error:

Unable to insert the virtual optical disk C:\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso into the machine MYHOST.

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 {b2547866-a0a1-4391-8b86-6952d82efaa0}

Here is a quick fix:
sudo apt-get update
sudo apt-get install virtualbox-guest-additions-iso
sudo apt-get install virtualbox-guest-utils

Once you are done with the above, restart the VM.