My ADHD/ADD boys both sleep like champs. They always have. They also napped during the day until they were about 4.
Here are a few tricks:
Have a bedtime routine. Put a pull switch in on her ceiling light so you can shut off the light and she can't get up at 2 a.m. and turn it back on using the wall switch (you can get an insert like this that you can just screw in to the light fixture: http://www.lowes.com/pd_72575-334-718V-SP-L_###-###-####_...= ). Remove any lamps she can turn on. Work to make her room as dark as possible. Remove all nightlights from her room. Children aren't afraid of the dark unless parents make them that way. Put up room darkening curtains to block light coming in from outside. If there is a TV in her room, unplug it or take it out. Remove all light and noise emitting toys. Close the door.
Put her to bed at 8pm (my boys were in bed at 7pm at that age). Make sure she stays there. Use the "Super Nanny" technique of silently putting her back in each and every time she gets out (NO TALKING TO HER).
Make sure she's warm enough/not too warm. A cool room and warm toasty blankets are best for sleeping.
Do NOT lay there with her. She needs to fall asleep on her own. And once it's YOUR bedtime, shut down the house and go to bed. Don't stay up. Turn out lights, turn off computers and TV's, and show her that you go to bed too. If she comes to you, silently get up and put her back into bed.
The very last resort: Turn her door handle around so the lock is on the outside. Lock it until she's asleep (which should happen quickly in that dark, toyless, uninteresting room). Once she's out, go in and put her into bed (she may fall asleep on the floor...no biggie). You shouldn't have to do this for too many nights before she figures out that she's supposed to be sleeping at night. Just be sure to unlock before you go to bed (especially if you're potty training).
The most important thing: Stay consistent. Don't look frustrated...it's like training a puppy. If you keep on showing her how, eventually she will realize what is expected of her.
Best of luck!