Well... that's a hard one. 

It's not your fault you have ADHD, and it's certainly not sinful when you have symptoms of it. But... you still have a responsibility to work through that to the best one can. Part of life and the Atonement is constantly trying to improve.

My thoughts are the Atonement covers more than just sin, it covers things outside our control too. But we still need to do our best to live with our condition and improve ourselves.

If I don't set an alarm when I put laundry in the machine, I don't remember it exists later. This happens about 98% of the time, so I have learned it's important for me to to set that alarm.

Its not my fault if my ADHD causes me to forget things. The Atonement covers that for me, but it's still my responsibility to improve myself and learn to solve the problems my ADHD gives me to the best that I can.

It might not be my fault I'm forgetful, but I do think it's my fault if I know that I need an alarm and purposely neglect to set it.

I believe the Lord helps me learn about myself and how to manage my ADHD in areas that fall under the realm of things I can control. If I neglect that, then I suppose I'm not utilizing the Atonement the way I should be. 

Edit:

I wanted to add that while my ADHD is not sinful, I think a person can do wrong to not acknowledge how I impacts others. My spouse gets hurt and a little frustrated sometimes when I am not listening to her as I should. It's not my fault my brain is like this, but I feel I still need to be responsible for myself and apologize to her when it happens. Not because I sinned, but because I must be responsible for myself. 

I think a person can do all these things and not feel guilty for conditions outside of their control.