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VirtualBSD is somewhat compatible with VirtualBox, but not right away and not easily -- not to mention the fact that we don't have the resources to maintain and seed two different distributions!
However, we've prepared these instructions to help you with the conversion process:
1) Download VirtualBSD-90.zip and unpack it;
2) Start VirtualBox and create a new virtual machine as described in the following screenshots (taken on Mac OS X, so don't be put off by the cosmetic differences);