There is no direct public bus from Luohu Port to HKIA. After passing the Luohu port, everyone should take the train to downtown HK and then transfer to the airport. Or, you could book a interborder bus supplied by these agent, which could drive you to the airport directly via Huanggang Port. here is one: http://ctsbus.hkcts.com/routes/air/air_shenzhen.html