It should detect the ethernet port automatically (and is generally active, so just fire up your browser) . Otherwise set it up using pppoeconf (sorry, forgot the exact term, refer to your Ubuntu thread )
Also, remember to keep networking on when setting up the Virtual Machine (or just enable it after setting it up)
It's been more than 8-9 months sice I used VirtualBox so I dont remember. I just recollect that I had enabled Networking while setting up the drive (along with the bootable image) and the virtual device (running Mandriva) automatically connected to the net on start-up.