I seriously doubt you could participate based on ADD alone. You have to meet the criteria for eligibility of the Disability tax credit. This means a doctor has to explain and sign off on your conditions. The CRA looks for significant restrictions on your ability to perform daily functions. Chances are if you can hold most forms of employment, don't require substantial therapy, can dress yourself, and can tie your own shoes you are not disabled by their definitions.
Many other mental health disorders are also not eligible. Mild or controlled PTSD, anxiety, and depression for example. You can try, but I would say it's a waste of time.
I know a guy that succesfully obtained the DTC status based on cognitive impairment and it was extremely difficult for him to do that. Took years of doctors, therapists, and testing. I believe he ended up being diagnosed with early onset Parkinsons or something like that.