Go to Tools > Online Web Registration > Field Options/Labels > Referral Information, and make your selection for How Did You Hear About Us to Required, Optional, or Hidden. The values that the customer can choose allow you to track your advertising and marketing effectiveness.
To add sources for the drop-downs, go to Tools > Edit Settings > Drop-down Lists > Family > Source (How did you hear about us?) and add rows as needed. You can track the results in Reports > Source/Referral Report.