How does one install the kube-dns addon for minikube?

The kube-dns addon should be enabled by default in minikube. You can run kubectl get po -n kube-system to check if the pod the addon-manager launches is there. If you don't see the pod listed, make sure that the addon is enabled in minikube by running minikube addons list and verifying that kube-dns is enabled

Edit: For me kubectl get po -n kube-system is a valid command, here is the output:

$ kubectl get po -n kube-system
NAME                          READY     STATUS    RESTARTS   AGE
kube-addon-manager-minikube   1/1       Running   2          5d
kube-dns-v20-7ddvt            3/3       Running   6          5d
kubernetes-dashboard-rn54g    1/1       Running   2          5d

You can see from this that the kube-dns pods are running correctly. Can you verify that your kube-dns pods are in the Running state?