http://inetpro.org/wiki/Using_QEMU_with_Olive_to_emulate_Juniper_Routers
I installed Olive on Ubuntu 8.04 64Bit using the above guide. Couple of straight forward instructions. The only thing you'd change is if you are using Xwindows to do the installation, you might not want to use the
-curses option in the Installing Olive Section
Also you need have ssh installed on your ubuntu machine to copy the junOS image onto your guest FreeBSD.