This is pretty simple.You have to have unlocked all four destinations from the Hong Kong Agency. This happens when you attain the 3-star level (thanks MastaGus for confirming that).
Once you have the four destinations from Hong Kong unlocked, simply keep playing the Excitement (aka PLAY, aka Agency) mode for the Hong Kong Agency until are four planes are full. Don't play the Travel mode even if some of the planes are full.
Once you complete the Excitement mode and the airplane seats are counted up to full capacity for all four destinations, the Achievement will pop.