Was looking for the same some time ago and found a thread on TripAdvisor. It was somewhat wrong though, but I then figured it out:
Procedure is:

Go to website http://www.ceair.com and look for "自助服务" which is one of the tabs. On the first line "号码" enter your ticket number and the next line "乘机人姓名" enter your name in the format "Last Name/First Name Middle Name" The website should find your reservation details now.

Look for 座位预留 in the left column and enter the same information again as above.

The top 'table' consists of four fields with a blue button between the first two. That is for the mobile and pin thing mentioned above (but doesnt work for non-Chinese numbers). Instead on the 3rd field, enter your email address. Once that has been done the red buttons on the right next to each of your flight can be clicked and you can select your seats.

Once selected, a small window will appear. Click on the left botton to "confirm selection".

Do not click "取消座位" as this is to cancel the assigned seat.