in case anyone is still looking:
there is a very simple game teaching the four basic operations:
Free online educational games for kids
not sure how ADHD kids would react to that, but there it's a slow-paced game...
I have two kids that suffer with ADHD and they battled with math. They just could not focus on the subject! They had no interest and were getting 20% for the tests. They were under review at school for this and it was a difficult time. I found that I had to try something new and went to the net. I found a few board games and they helped.
Have you found other ways to help?
Derek from funkidmathgames.com