I have downloaded it via synaptic.
Not to confuse you further
silverbirch 
, which Virtualbox did you install?
There are 2 versions - the OSE (Open Source Edition) version that installs using Synaptic in the regular way.
The second method is using the "
getvirtualbox" script - which is again downloaded in the usual way from Synaptic, BUT, with the "
getvirtualbox" version you then have to run the getvirtualbox script to actually install it.
The
getvirtualbox script works in a very similar way to GetOpenOffice - it pulls all the necessary files down and installs it for you.
The
getvirtualbox one is whats known as the PUEL (Personal Use and Evaluation License) version and I believe is the one most user's install.
To run the
getvirtualbox version (once it's been installed from Synaptic) go to your main
KMenu >> More Applications >> Emulators > and run it from there. Give it some time - you'll see a Message Window running - and when it's all finished you'll then have a second entry in the
More Applications >> Emulators > menu from where you can run Virtualbox.
If any of that is unclear

please post back.