What Nancy said is correct. TENS cannot be applied to the head or face at all. In fact, I've never seen TENS used for Migraine, only for tension-type headache, and even then, you need to take great care and ask your doctor about the placement.
We're sorry we can't help you with this, but your doctor needs to be instructing you. A TENS unit is prescribed by a doctor, he knows the best placement for your particular medical issues.