Like most others, i had problems getting my Ninja to kick. I stumbled upon it while diving in the ocean. Having a rebreather equiped, i noticed the kick text message. I took this a step further and equiped other items in the 'shield/second weapon' slot. Having a second weapon equiped allows a swing and an attack with that weapon, but no kick. With a shield equiped, i swing and kick on just about every attack. Hope this helps someone getting a kick out of thier martial arts.

Wanted to add...
If anyone kicks without something equiped in the off hand, drop a line and let me know your secret. I dont mind getting 2 attacks with a shield equiped also mind you, but it would be good info for future characters.