I've booked on Air China via AC before, and you CANNOT do seat selection on Air China's website.

Air China only allows you to do seat selection for eligible tickets booked directly on their website - third party or rewards cannot be done online.

As mahasamatman identified, you must call in to do so, and have a few drinks or other relaxants first. If you're on hold, they require you to re-validate that you wish to stay on hold every 30-60 seconds, by pressing a specific button, and listening to a 20-second automated message each time.

It almost drove me crazy... but I wasn't going to take chances with my seats.