After you leave China the second time is when The Missing Link starts, you can play it at that point if you want everything to play out in the "right" order.
I loved TML, and I'm glad they did it as a separate gaiden-game style story over an integrated one. The boss fight in particular was good, as one developed by Eidos means it's not a straight-up shooter and you can use your specialized set of skills to defeat them. The DLC stuff already present is good enough, though I do wish they could complete some more areas of the game and release those as integrated content - the game could've been that much more amazing if it had the extra hubs they'd planned.
Now I would have liked to have seen this auto connect into the game. So if you had it and when you left in your little cryo bed. It should have kicked in there!
So to basically say when to start playing it is then! Not before or after. If you want to try and keep in with the story time line.