I used to do alot of gymnastics, and still do open gym alot which has really helped my upper body strength.  I did the muscle ups on a short-ish wall (between 5 and 6 feet) that is outside around my neighborhood. 
As wierd as it sounds, it seems like this has most of the same muscle movements as the rings in gymnastics, which it odd because they are free hanging, but it feels similar, just that I have more support on a wall.

I know I need a more powerful pullup, so I need to start adding weight when I do pullups, but after I notice a significant difference in strength, what's the easiest way to transition myself into muscleups?  I've tried jumping into them and I can't do it on anything higher than my head.  Should I try kipping on a bar?

Is it easiest to start out on a bar?

with kipping a bar will be easier than rings... deadhang bars are the hardest out of all of them though.

Okay, with *good* technique, deadhang bars are harder than deadhang rings. With deadhang bars you have to displace yourself behind the bar cause you can't go through it which takes more energy than stabilizing the rings. I can tell the difference because I almost never do either type of muscle up (heck, I don't do really any muscle ups) and deadhang on the bar is always a few less than on rings.

Relearn with slow negatives then explosive pulls.