If ADHD is a concern for you - concentration is obviously a (often frustrating) factor as well, when it comes to meditation. People with ADHD have a hard time concentrating, and because their brain not only has a harder time concentrating, but it also can filter out the thing it's being asked to concentrate on, as if it were a distraction.
If you're working with a focus oriented type of meditation, this may cause frustration, and a feeling that it isn't working for you - and it may not actually be doing you much good.
It does stand to reason - that using a focus based meditation might help improve the ability to focus in those who are impaired in this area - but what if the person just gets frustrated because it is so much harder for him or her than other people?
We know that being frustrated can hinder a meditation practice. From this we can conclude that it may be better for folks with ADHD to try a practice that doesn't rely on focus as a method of meditation.
Studies also show that you can attain a higher state of relaxation when you're not trying to focus on something - which is the case with NonDirective meditation. So while doing this, we learn to let thoughts pass rather than actively trying to get away from them. This state of relaxation, and allowing can be extremely beneficial for folks with ADHD because the eliminated stress can allow the brain to function more optimally.
Brain wave studies seem to echo this theory. Individuals with ADHD tend to have much higher Theta brain waves - which in normal people will help block out irrelevant information. The higher theta values seen in ADHD people are thought to be blocking even more information - sometimes including the information that the individual wants to concentrate on.
Nondirective meditation increases Alpha brain waves, which are associated with an alert state of focus - kind of like when you're ready to work and you're in the zone. This may be a more beneficial form of meditation for this reason.
As we practice nondirective meditation more and more, we become used to operating in this zone, and the brain learns more and more how to relax and maintain that "restful altertness".
At the same time, we're experiencing more and more relaxation, and less and less stress. This helps bring a sense of relaxation and calm to our daily lives, which can help us to find the focus throughout the day as we need it.
As an ADHD person myself, I find that NonDirective meditation was the only type of meditation that I was able to stick with long enough to see any of the benefits.