minikube: Driver "virtualbox" not found

I’m trying to experiment with minikube but can’t start the VM.

$ minikube start --show-libmachine-logs
Starting local Kubernetes cluster...
E0727 20:53:10.102359   65464 start.go:78] Error starting host:  Error creating new host: Driver "virtualbox" not found. Do you have the plugin binary accessible in your PATH?

I’m not sure if I have the virtualbox driver installed but docker-machine can’t find it, or if it’s not installed at all.

I’ve installed docker-machine 0.7.0 via brew. I’m running minikube 0.7.0. I don’t see the virtualbox driver installed via brew:

$ brew search docker-machine
docker-machine ✔                         docker-machine-nfs
docker-machine-driver-xhyve              docker-machine-parallels

I’m using virtualbox 5.1.2.

Thanks in advance.