Nimble Fingers is located in the Sub-Sewers beneath Freedom city. You cannot reach the mutation site without first defeating the bug queen, so find an escort or ask a corpmate for help, or be prepared to defeat her yourself. (Unlikely due to the kind of build you most likely have if you're going for this mutation)
This mutation grants:
This is the only mutation that positively effects your craft and locksmith abilities, and is one of only two mutations that raise reflexes (the other being twitchy nerves). It also is "easy" way to get the ability (the other methods being carrying a heavy massage table around with you and trying to get Empath).
The minus 2-4 beat protection can be a VERY painful handicap early on when many things do nothing but beat damage (it is HIGHLY recommended high density and possibly lithodermis to offset this). Furthermore, the -2 Endurance means the user cannot fight as long, along with handicapping important skills such as swim and climb. Taking this mutation also prevents the user from taking ripper and therefore from taking Abomination, but chances are crafter types wouldn't take either of those due to the minuses to crafting, -2 brains, and mental illness issues.
Massage lets the user massage other players in order to reduce their stress levels and it should be obvious that you won't be able to massage yourself. This is done by using, or possibly for more specific treatment. Massaging will also give the user Medic ip for massaging players with stress on them.