The PBI for version 4.1.8 just finished building and I am testing it at this very moment. Generally a PBI lags a few days behind the FreeBSD port updates since we have to build the PBI, test it, fix any issues, and repeat if necessary before it gets approved and pushed out to the AppCafe.
The PBI sets up PCBSD as the host environment for virtualbox, as opposed to the guest environment (the OS to be run inside virtualbox). The vbox-guestadditions are already setup for the host environment within the PBI, but they also need to be setup in the guest environment in order to be properly utilized. To accommodate this, we provide a pre-built virtualbox install (.vdi) of PCBSD with all the guest additions already enabled that can be downloaded from www.pcbsd.org
. If you are trying to run something OTHER than PCBSD in virtualbox, you will need to find the instructions for how to enable the guest additions for that particular OS.
If you have any further issues with getting it to work, feel free to send an email to the PCBSD pbi-dev mailing list about it. There are many other people on there that are more familiar with customizing Virtualbox and can help you out much faster than I can.