Today I’ve updated my VirtualBox to version 4.1.16-78094. When I tried to install the new version of Guest Additions, I got the error message
Unable to mount the CD/DVD image 'C:\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso' on the machine. Would you like to force mounting of this medium?
Could Not mount the media/drive 'C:\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso' (VER_PDM_MEDIA_LOCKED)
as you can see in the picture 1 bellow
Notice that clicking in the Force Unmount button has no effect.
The solution is simple: unmount the CD/DVD driver.
After that you will be able install the Guest Additions.
Just as a reference:
My host: Windows 7 64 bits sp1
My Guest: Ubuntu 12.04 32 bits.
Hope it helps.