Where's my ephemeral storage for EC2 Instance

RTFM - I found my answer in the Amazon EC2 Instance Store docs.

Instances that use Amazon EBS for the root device do not, by default, have instance store available at boot time. Also, you can't attach instance store volumes after you've launched an instance. Therefore, if you want your Amazon EBS-backed instance to use instance store volumes, you must specify them using a block device mapping when you create your AMI or launch your instance. Examples of block device mapping entries are: /dev/sdb=ephemeral0 and /dev/sdc=ephemeral1.

So in my case, since I choose an EBS root device, I'll need to rebuild the instance with the instance store volume.