virtualbox.org

My hardware: Apple iMac 2.66 GHz Intel Core i5
My host OS: Apple Mac OS X 10.8.3
My VirtualBox: 4.2.10
My Guest OS: openSUSE 12.3 (Awesome, by the way!)

Command attempted: From VirtualBox menu bar Devices-->Install Guest Additions .... (Host+D)

ERROR RESULTS APPEAR IN A DIALOG BOX, TITLE: VirtualBox - Question

Unable to mount the CD/DVD image
/Applications/VirtualBox.app/Contents/VBoxGuestAdditions.iso
on the machine openSUSE 12.3. Would you
like to force mounting of this medium?

Could not mount the media/drive
'/Applications/VirtualBox.app/Contents/VBoxGuestAdditions.iso'
(VERR_PDM_MEDIA_LOCKED)

Details:
Result Code: NS_ERROR_FAILURE (0x80004005)
Component: Console
Interface: IConsole (MAC address given here)
Callee: IMachine (MAC address given here)
-------------------------------------------------------------------------

Notes:
I get the same error if the openSUSE installation DVD with iso image is in the iMac DVD drive or not.
I was able to Install Guest Additions with no problems on the same iMac and VirtualBox with Fedora 18.

With the guest shut down (not saved state) remove the install media from the virtual drive and it should work when you start the guest and select install guest additions.

I am probably misunderstanding your instructions. I used Virtual Box File --> Virtual Media Manager ... --> Optical disks Tab. I removed the listing for openSUSE there which was marked in red and was marked with a red !. I restarted openSUSE and tried to Install Guest Additions. I got the same error as before.

Now when I look at the optical disk settings in VIrtualBox I see the same thing listed for Fedora18 and openSUSE 12.3:
VBoxGuestAdditions.iso 54.74 MB
Location: /Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso
Attached to: Fedora 18, openSUSE 12.3

No not the media manager.

Shut the guest down and then in the main manager click setting for the guest and go to storage. Click on the CD (at the right) and select to remove from drive.
Now start the guest and click the device tab at the top and install guest additions.

This should help Howto: Install Linux Guest Additions