Hey Ruben,

from my past times using VirtualBox I found this quote:

“This one: It took me over a day of sweat, blood and tears to get the network running. Actually the PCNet adapters don't seem to work at all. I found just this one Network setting to be good:
Adapter 1: Intel PRO/1000 MT Server (Bridged adapter, eth0)
where the `MT Server' version is absolutely essential, as the MT Desktop version does not work!”

from 

http://www.plan9.bell-labs.com/wiki/plan9/installing_plan_9_in_a_virtualbox/index.html

I would recommend using qemu these days, though.

Cheers,

ingo