Unable to mount the CD/DVD image C:

[GuestAdditionsAFU]

When I do the menu entry for Devices->Install Guest Additions I get this error dialog:
VirtualBox - Question
? Unable to mount the CD/DVD image C:
\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso on the machine LinuxMint12KDE. Would you like to force mounting of this medium?
Could not mount the media/drive 'C:
\Program Files\Oracle\VirtualBox/VBoxGuestAdditions.iso'
(VERR_PDM_MEDIA_LOCKED).

> Details
Result Code: E_FAIL (0x80004006)
Component: Console
Interface: IConsole {1968b7d3-e3bf-4ceb-99e0-cb7c913317bb}
Callee: IMachine {5eaa9319-62fc-4b0a-843c-0cb1940f8a91}

[ Force Unmount ] [ Cancel ]


I have no idea what the problem is. Using this menu item seemed to work on another Virtual Machine. If I hit the [ Force Unmount ] button the dialog box goes away. What I want to happen is for the OS to use the entire screen rather than just a square in the middle of the physical display. Apparently Guest Additions is somehow involved in this occuring (which really is baffling to a layman - why doesn't the virtual machine just look at the hardware and present the guest OS with an emulated display the size of the physical display minus the window decoration??) but I still have only a square in the middle of the screen being used, even after doing the menu item and stopping and starting the virtual machine. I did somehow manage to get a full screen display when I run Linux Mint 12 GNOME in virtual machine (this one is Linux Mint 12 KDE).

[Perryg]

You need to eject (unmount) the install CD from the drive, then click on install guest additions.

[Praveen]

perfect!

Thanks a lot