I just installed elementary OS 0.3.1 in a VirtualBox 4.3.12 (using MacOS X 10.9.5 as a host), installed the guest additions (I had to mount the CD manually and execute the .run script manually) but the screen resolution is still at 640x400. The vbox kernel module seems to be loaded:

 johannes@johannes-VirtualBox:~$ lsmod | grep vbox
 vboxguest             249856  5 

Am I missing something obvious? Should I maybe upgrade my VirtualBox host software?