For those who are wondering, the inciting incident is The Thing That Happens that makes your character move from their place of safety into the story world. In The Hunger Games, it's when Katniss volunteers as tribute in place of her little sister.
The disturbance is something that's messing with your main character's normal existence. Going back to The Hunger Games, Katniss and Gale are enjoying a normal afternoon of hunting ... only it's not a normal afternoon because the reaping is about to happen and they're nervous.
The "doorways" James Scott Bell describes are two points of no return in the story, transitions from the beginning to the middle, then the middle to the end. The first doorway takes the reader from the character's normal, albeit "disturbed" world, into the story. The second doorway occurs leads to the final battle. For our purposes, today we'll just discuss doorway number one.
Your character will most likely need a push out of their normal world and into the story. This is a doorway that they should decide to walk through. How much better is The Hunger Games because Katniss decides to go rather than just getting drawn? Suzanne Collins had an easy way to get Katniss in the games (she'd already established that Katniss's name was in the lottery tons of times) but her choice of a "doorway" - Katniss choosing to take Prim's place - makes for a much stronger story.
James Scott Bell says when you're trying to figure out if your character has gone through the first doorway is, "Can my character walk away from the plot right now and go on as he has before?" If the answer is yes they can, you haven't gotten them through the door yet.
When should a character walk through that first doorway? The popular opinion is the 1/4 mark, which I won't disagree with. I could be wrong, but I think Katniss volunteers for the games at the end of the first chapter, and I think that works great too. It'll depend on the overall pacing of your novel, I feel. A book like The Help might require a bit more time to get to the doorway than something like The Hunger Games.
In your manuscript, does your character have a doorway to walk through? If so, are they choosing it or are you the author shoving them through?