The problem with ADHD is that it doesn't really have its own trademark indicator, the way that schizophrenia or Asperger's might. There are several different subtypes of ADHD, and they tend to look different from each other, particularly in the instance of ADHD with hyperactivity and ADHD-PI (primarily inattentive). Even then, there are plenty of disorders that mimic the characteristics of ADHD (such as histrionic personality disorder or sluggish cognitive tempo), and so it is really difficult to tell whether or not a character has ADHD, a mood disorder, an anxiety disorder, or something else entirely.
Furthermore, movies like A Beautiful Mind are autobiographical in nature and tend to focus primarily on the disorder and its deleterious effects on an otherwise prominent individual. This is difficult to do with ADHD, because the symptoms of ADHD are often mistaken for laziness, naïveté, sluggishness, rashness, and a variety of other negative personality traits. On the other hand, disorders like schizophrenia are rarely mistaken for personality traits. It is very clear (in the case of John Nash) that there was something going on almost unrelated to the man himself. Take the description of the movie for example:
The story begins in the early years of a young prodigy named John Nash. Early in the film, Nash begins to develop paranoid schizophrenia and endures delusional episodes while painfully watching the loss and burden his condition brings on wife Alicia and friends.
You would rarely hear such a description of an individual with ADHD, because ADHD is often early-onset, and so the difference between one's natural personality and a neurodevelopmental disorder is not always clear.
On top of that, characters are often portrayed as having symptoms of ADHD, but it is considered to be a part of their personality used for comedic effect. Other times, their ADHD-like characteristics may be attributed to other disorders, such as the case of Cat Valentine in the Disney Channel show 'Victorious', who suffers from bipolar disorder (as per the show's storyline) yet her characteristics are speculatively more along the lines of ADHD, or even dependent personality disorder. This opens up a can of worms to debate against, however, as characteristics can only serve as implications of a disorder, and are not enough to diagnose a person in general. Furthermore, even a professional cannot diagnose a character and call it a day, as characters are under the artistic license of the producers and thus there is nothing definitive a professional can say about them.
In terms of characters, it is speculated that Tigger from Winnie the Pooh has classic ADHD (as seen here, though this is largely meant to be humorous). It is also thought that Winnie the Pooh himself has primarily inattentive ADHD. The character Tony Stark of Iron Man is so highly speculated to have ADHD that I have heard him being used as an 'example' of a superhero with ADHD, so that children who struggle with the disorder can feel more confident about their abilities.
In terms of confirmed characters, I know of very little. After further research, I know that the character Lucy Knight of the show 'E.R.' was on Ritalin (a medication used to treat ADHD). I have also repeatedly read that Luna Lovegood of Harry Potter has ADHD-PI, but I think that her character is too broad for any diagnosis. I don't know if Bradley Cooper's character in Limitless has ADHD or not, as the whole plot seems to be akin to the effects of someone unintentionally taking Adderall.
I know that this isn't a movie or show, but Calvin from the comic strip Calvin and Hobbes series is confirmed to have ADHD, as per the creator of the strip and as demonstrated in this comic.
EDIT: Forgot to mention this, but the character Michael Scofield of Prison Break is diagnosed with low latent inhibition, which is often used interchangeably with ADHD.
