Since you're pretty sure it's tendonitis, I can give you some suggestions.
You probably know that "-itis" means inflammation/swelling. And it's a symptom of something, not the 'something' itself. There is an underlying cause. You can reduce/eliminate the cause and/or reduce the symptoms.
For the cause:
For now, I would suggest backing off on step and doing rebounding or your eliptical (try "Cardio Coach" on the eliptical to make it less boring).
When you get back into step, don't do it every day (you didn't mention how often you are doing it, but it seems that some people can't do more than 2x a week without problems. YMMV, so you have to see what works for you).
Watch pivoty moves (I personally find that they are tougher on my knees than impact itself), especially moves where you go from the side of the step to the back. I just did Low Max, and there are several moves where this happens, and where I have to be careful to prepare the 'behind-the-step' move by stepping back far enough after the side move, otherwise I'm in a position that makins torquing easy. And Cathe doesn't say anything about this or remind us to remember to take a bigger step this or that way).
What kind of surface are you stepping on? The absolute worst is carpet over cement. The carpet doesn't cushion the hardness of the cement enough, and it makes pivoty moves more of a risk for torquing the knee. Puzzle mats are good for absorbing more impact, and for allowing an easier turn/pivot.
Make sure to work the muscles around the knee so that they are balanced, and stretch them well. Make sure especially to stretch the hamstrings, hip flexors and IT band.
For symptoms: you can ice the tendon after a workout (about 20 minutes of icing from either a bag of peas held on the area, or an 'ice massage' with a dixie cup filled with water and frozen). You can also take anti-inflammatory supplements. Flax is an anti-inflammatory (buy whole flax seeds--golden flax are tastier and also a bit more nutritious--grind about a cup at a time and keep it in your fridge and add 1 Tbsp. to a smoothie, or sprinkle on oatmeal or cereal). I take a supplement called "Blue mangosteen" (from
www.rawfood.com) which is made from Klamath lake blue-green algae and mangosteen. It acts as an anti-inflammatory.
HTH