(Note I also posted to cdh-users)
My reply from cdh-user, cross-posted here:
I've downloaded a fresh copy of the VirtualBox appliance from the website and VirtualBox Guest Additions are still working for me. Are you sure you downloaded the VirtualBox option? The *.vmdk and *.qcow2 disk images included in the VMWare and KVM downloads are also usable in VirtualBox VMs, but then you'll be missing some of the VirtualBox-specific networking setup, hardware configuration, and obviously the Guest Additions. Only the *.ovf file will do all that for you in VirtualBox.
Also check the SHA-1 hash of the 7-zip archive (cloudera-quickstart-vm-5.0.0-0-virtualbox.7z) to confirm it's the correct file and did not get corrupted during the download. It should be d18da24ec936f6a5e518d1dcdbde772dfc04c908 (you can find all the hashes on the downloads page). If all of that looks correct, I would suggest either installing the guest additions manually (using the kernel packages I mentioned above), or downloading a fresh copy and try again. Also, please let me know if you confirm that this is the correct image. If there's some kind of occasional failure during boot-up that prevents Guest Additions from working as they should, I would like to look into that some more. However, it's been working great for me, and I'm using the exact same version of VirtualBox as you (on Fedora Linux), so I would hope it works for you!