Kids with ADHD struggle with self-activation, and it shows up largely as “lack of motivation!" Without a strong motivator, it's hard for ADHD kids to get ANYTHING done – sometimes even if its something they really want to do.
While there are thousands of strategies, the most important thing to understand is that, generally speaking, kids want to please. They want to be successful. They're not lazy, crazy or stupid. It really IS VERY hard for them to get activated, and then to focus, and then to sustain effort.
One simple homework assignment requires a huge amount of executive function. So it's NOT easy, or they'd probably just do it.
As a parent it's hard to remember this. After all, sometimes kids really do LOOK like they're being lazy when they won't get up and take care of a simple task or request. But whenever you ask your child to do something “simple” and you're met with resistance, ask yourself, “what are the steps involved?” or “what part of this might be difficult?” Just a little understanding – helping to break simple tasks down into smaller parts – will help your child learn to take on the simple stuff. Who knows – she might even get to please you in the process!