Currently this requires downloading and using a custom iso. Let's make this process as friction free as possible by automating as much of this via wharfie as possible.
Here's the existing instructions https://github.com/mozilla/wharfie#updating-boot2docker-with-virtualbox-additions-to-support-volume-mounts-osx-hosts
This should be fixed with docker 1.3 there's some wip docs here: https://github.com/mozilla/marketplace-docs/pull/29
Merged and worked for me.