I have to say I never found TENS machines any good for e-stim, the frequencies and pulse patters just weren't right for me. I did eventually invest in a proper e-stim box (the PES one, although many people claim others are better that one offered the right price/performance ratio for me).
In terms of electrode placement that depends what the object is. For erotic teasing I find an electrode on the cock shaft and one at the base of the scrotum works well and doesn't provide enough stimulation to bring me to orgasm. The box CAN produce (and even force!) an orgasm, indeed it is sometimes used in place of traditional milking while I am locked. For this I am normally bound over a bench or stool with an electrode placed onto the head of my cock (it has a short spike which is inserted into the urethra - see
www.peselectro.com/pescontent/images/123-pes.jpg). A second electrode is inserted into my anus (usually one designed to specifically target the prostate -
www.peselectro.com/pescontent/images/main-prostate-pic.jpg but a standard electro butt-plug works just as well. It takes some time to work up to the power level required to produce an orgasm, however using this configuration I have never failed to release, even when strictly ordered not to and trying my hardest to resist.
Since I was locked in my new CS-100 metal belt She has slightly changed the way She uses the box. Now, she simply squirts the tube full of conductive lube, attaches a pad to the outside and puts the prostate stimulator in my ass as usual. This way She is able to force my balls to empty themselves without ever removing the belt and without having to perform a manual milking. She prefers it this way and it has largely replaced 'normal' prostate milking while I am locked.
HTH,
ScottishSubby