I have adhd, and flow comes naturally for somethings but it's rare. Usually I will get it in intellectual or creative endeavors. It is great when I am learning a new bit of mathematics, or when I am programming. But the absolute worst thing about it is that the smallest piece of distraction will completely derail me. Medication helps a lot in this aspect, but it's unavailable where I live right now. I used to meditate to help but for some reason I stopped. One of the best parts about "flow" is recognizing when it's happening and then being able to keep it going.
Funny, I am ADD as well, and was medicated most of my life. I chose to get off medication because I didn't like the way it made me feel. Too intense at times. Daily meditation is what keeps me on track and focused. That and lots of coffee (stimulants obviously help).I used to meditate to help but for some reason I stopped.