What do you do when a seven year old with add/adhd has a temper tantrum?

Have a plan. There is nothing unique about adhd in terms of handling temper tantrums--excpet they may be a bit more prone to them. To little space to go into detail, but look up the explosive child or others sources about dealing with out-of-control behavior. Develop a consistent intervention that keeps the child safe and minimizes secondary gain (benefits) the child might be getting. Adhd is not an excuse.
Same as any kid. Temper tantrums are effusive expressions of frustration intolerance. If they are infrequent best advice is to ignore it, give the child a chance to calm on her own. If they are frequent, then there may be problems, including adverse reaction to the medication, an underlying comorbidity, or behavioral disarray. If latter, psychologist & prescribing doc need to be involved. Consult doc for help.
Pitching a fit? A temper tantrum is a temper tantrum.Children with adhd tend to be more immature, and have more difficulty with adaptibility;but the behavior and management is no different.Avoid situations that easily frustate the child, avoid providing either positive or negative rewards for the behavior, avoid getting angry, seek professional help if behavior persists or worsen.A behavioral phychologist could help.
Walk, don't talk, to. To time-out. Neurotypical 7 yo children care about performing tasks well & know what to do to make others think well of them. They tend to reject disruptive kids who have delayed prosocial & coping skills. Behavioral therapy teaches him to recognize his "rumbling stage" then self-calm & teaches you immediate consequences like effective time-out & a token economy system for rewarding self-calming.