What is it that you are trying to stim? And what is the goal of the stimulation?
I feel a little unsure about just giving settings... I'd feel much more comfortable if I could see the unit and placement of the electrodes... Feel free to email me pictures or video!

For strengthening, which is a common use of estim, here is how I would set up a unit on muscles that you can turn on but maybe can't move the joint through it's full range:
phase = 300usec and frequency = 35pps then turn the amplitude up slowly until the muscle contracts stronger than you can do on your own. This is meant for you to work WITH it.
I would ramp up for 2 seconds, on time of 4-5 seconds, ramp down for 2 seconds, then off for 20-25 seconds. This is a 1:5 onff setting for rest in between reps. And since it is strengthening, you do want to fatigue the muscle. If you notice the contraction isn't as strong after a few reps, you are done for the day. If you can make it to 20 reps, congrats! But I wouldn't do more than 20. Just like weight lifting, take a day off in between to let the muscle recover and strengthen.

Hope that helps. PLEASE ask for any clarification!!