Hi Here is an excellent thread on diet and nutrition, http://talkcockatiels.com/showthread.php?t=27479, and cockatiels love vegetables, heres the safe veggies thread, http://talkcockatiels.com/showthread.php?t=536.
Be patient and try different ways of presenting the food to her as cockatiels can be picky and wary of new foods, you can try hanging greens through the bars of the cage, or offering some veggies to her by hand, or chopping them up small and giving her a bowl.
Just a few added thoughts.. some birds like their veggies a certain way. Some enjoy them clipped to the side of the cage.. some like them cooked.. some like them raw. Some birds prefer their veggies chopped up really fine and others like bigger chunks. Some birds will eat them in a bowl at the bottom of their cage.. others in another dish hung beside their seed/pellet dish. It's an experiment. Just don't give up if you try once and they won't eat it. Sometimes you can even try the veggie the exact same way and then after a few tries they decide they like it. Patience is key... and figuring out what your bird enjoys.
Have fun experimenting! Really, as DyArianna said, they like them a variety of ways depending on their tastes so it is YOUR job to find out what they like! It is a bit of work but it's worth it once you see them enjoying it. TIP: pretend to eat it with them and "enjoy" it too - make a lot of appreciative noises and make them jealous that you're enjoying it! They'll get curious enough! They always want what you've got!
A big thing for my tiel too is texture. He does not like mushy, sticky things- so rice, pasta, frozen vegetables, he won't touch. So I think you just have to keep trying to see what your tiel likes and doesn't. It can take a little while but keep trying.