PS: there are no such packages installed here... So I suppose there is nothing special to do. Thanks again.
Here's the easiest way to do it. With your guest VM running, click on Devices > Install Guest Additions at the top of the VirtualBox Window. In most cases, the guest OS will automatically mount the guest additions CD.
Then, in the guest OS, just browse to the CD's mountpoint. You'll want to run, as root,sh VBoxLinuxAdditions.run
If it shows the GuestAdditions already installed, proceed anyway. After rebooting the guest VM, the video will be right and the mouse will be integrated with the host's.