I think the problem with using a TENs machine is that you are starting off without total faith in your body's ability to birth comfortably, so I would be more inclined to advise you to work on your belief's and fears more with your practitioner.
To be most effective TENs have to be used right from the very very start of labour, you have to make sure it's a proper obstetric one with a "boost" button. Then during each surge you either have to press or turn a dial on the boost button, in my experience it is not possible to do this as well as focussing on slow breathing and really deeply relaxing. So the only possible way I would advise use is if the controls are handed over to the birth partner, the problem with this then becomes will they really know when you're in a surge - i.e: when to press the button! Also you obviously can't use them in a birthing pool.
Our light touch massage technique works on much the same principles as a TENs machine so I would read up on that again and get your practitioner to demonstrate again if you don't totally understand it.
Certified HypnoBirthing Practitioner, Doula, Hypnotherapist & Homeopath
HypnoBirthing classes in Cambs, Essex, Herts & Suffolk.