I thought the 2D and 3D acceleration support for guests was only in the PUEL edition of VirtualBox, and not in the OSE version. (I'm likely wrong about this, I'm much more familiar with the PUEL, not with OSE version of VirtualBox.)
Either way, I'd install the guest additions that come with your exact edition/version of VirtualBox, not what happens to be in Synaptic. Click on Devices->Intall Guest Additions. Once you click on Devices->Install Guest Additions, if that doesn't make the VBox CD image available on your guest desktop, reboot the guest. Once the guest additions show up, here is how I install them:
If your guest is running the 32-bit version of Ubuntu, you'd instead run the following:
...followed by a reboot of the guest.