Ok, there is a mission called "Knowledge of the Past" that you must be
offered and accept in China. I believe you can only get this after you've done the mission chain where you get a vision inside the Dragon's Maw etc.
"Knowledge of the Past" will then bring you to France, where you must go through Chateau du Landgraab, one of the most advanced and complex tombs in the game. Inside, you will eventually find the keystone (in the part of the chateau you unlock with the keystone you get earlier in this mission chain). The chain then brings you back to China, where you will have to plow through the rest of the Dragon's Maw.
If you want to read some more about quests in China look into Carl's guide, it's an excellent source for this sort of information:
http://www.carls-sims-3-guide.com/worldadventures/china/If you are not getting this mission, then you may just have bad luck. I am not aware of any way to determine exactly what missions you will be offered, so I guess you'll just have to be patient, it'll show up on the adventures' board eventually.