What are my right regarding slander at work?

The employer is not responsible for what this employee is doing IF you do not have ADHD or some other condition: that is because if you do not have ADHD, then this is defamation, or a mistatement of material fact (basically, a lie) which damages your reputation. Committing defamation is not part of this person's job--it is not what they are employed to do. Since it's not part of their job, the employer is not liable or responsible for it. The person defaming you, however, is liable for his statements, and you could sue him for this--or start by letting him know you will sue if he continues.
If you do have ADHD or some other medical condition causing symptoms which he is mistaking as ADHD, then this is disability-based employment discrimination or harassment: employees may not be harassed for their medical conditions. The law requires employers to take steps to prevent or stop such discrimination or harassment once they become aware of it. Since you have made your supervisor aware of of the situation, if the supervisor or someone else in the company (e.g. HR) will not take reasonable steps to stop this, the employer may become liable for disability-based discrimination or harassment. In that case, you could contact the federal EEOC or your state's equal/civil rights agency to file a complaint; the agency can investigate and take steps to stop this (and sometimes, in egregious cases, get you some compensation, too).