Unable to mount GuestAdditions

[nbogs]

Host:
Core 2 Duo MacBook Pro with OSX 10.6.8

Guest:
Ubuntu Server 12.04 LTS
Base Memory: 612 MB
Boot Order: Floppy, CD/DVD-ROM, Hard Disk
Video Memory: 12 MB
Remote Desktop Server: Disabled

IDE Controller: IDE Primary Master (CD/DVD): VBoxGuestAdditions.iso (49.75 MB)

SATA Controller: SATA Port 0:
Ubuntu Server.vdi (Normal, 8.00 GB)

Error:
"Unable to mount the CD/DVD image /Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso on the machine BackTrack. Would you like to force mounting of this medium? Could not mount the media/drive '/Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso' (VERR_PDM_MEDIA_LOCKED)."

I've done some googling, but all of what I've seen has been for Linux/Windows hosts and hasn't been of any help. I have installed dkms and gcc, and I've tried the "Install Guest Additions" option under "Devices" both with and without having the .iso image mounted. All the same, I always get that error. Any ideas?

Thanks!

[ChipMcK]

What happens when/if you reply 'yes'?

[nbogs]

The only options available are "Cancel" and "Force unmount" and upon clicking "Force unmount" (yes, unmount, not mount) the Guest Additions error window closes but nothing happens. After clicking "Force unmount" and then clicking once more on "Install guest additions", the same error arises.

I suspect that the installer may be trying to unmount the disk and then proceed with the installation (as one of the solutions I came across suggested).

[ChipMcK]

in the virtual machine, unmount whatever cd/dvd is mounted first
Then Install the GAs