If I remember correctly, I believe the passenger seatbelt has to be buckled too. The manual says anything over I believe 70-75 lbs seating in the passenger seat should illuminate the passenger airbag light on the HVAC basel trim. I remember sitting some heavy package (90 lbs) on the passenger seat but no light. Whenever my daughter or neighbor's kid sit in the seat with the belt buckle, my passenger airbag light is always lit.
Your light should work....if it doesn't I'd return to the dealer just to make sure it is not a malfunctioning weight sensor in the passenger seat.