I would like to speak to the manager about an incident that happened at the store last night. How can I reach him?

Go to the store or call. Going in is a more professional look.. make some quick notes to what you want to tell him so you don't forget. Being polite I will give you better response back

Go to the store or call and begin with customer service. Request an audience with the manager.

Yes, talking with someone at Customer Service is the way to begin.

Go to customer service and ask for the manager or call and ask to speak with the manager.