Here is a handy adapter that will allow a TENS unit with ordinary TENS pin leads to be converted to a 3.5mm audio jack adapter.  If you happen to have or want to purchase a 3.5mm lead wire to use with your TENS unit, this adapter will allow for that as well.

Converts any TENS lead wire with standard 2mm push pin plug into a 3.5 mm jack, allowing the use of all 3.5 mm plug accessories.  The polarity of the TENS lead jacks are color coded (black and red) for easy identification. Measures 6.5″ in length.