Give her time and keep on trying, your being impatient already as it hasn't been that long since you got her. During the beginning I don't push the change, I offer it to them and if they eat it great, if they don't I'll give them time to get settled in before I make any changes. I usually start making the change after a month of being with me. But that depends on the bird, if they take to it right away then I make instant change. I give them whatever they were being fed in their last home for the time being. Then when I start, I give a bowl of pellets that's available all day everyday. Then a bowl of fresh fruits and veggies in the morning. Then in the evening time I give a table spoon full of seeds. Next day I start all over again. During anytime you see them start eating pellets real good, yank the seeds and don't give it to them anymore. No nuts or treats during this time!!!! Let them be on the new diet and after they've been on it for a month or two, I would start offering treats and nuts again.
I would highly suggest you not having your cockatoo on your shoulders! Especially since she's not used to you at the moment nor are Cockatoo trust worthy to be on shoulders. She needs to learn early on to entertain herself if you can't be with her. It is detrimental to them if you do what she wants you to do. That's when people start having issues with Cockatoos.