i recently bought a pay-as-you-go china mobile SIM card (for 100 yuan, or around $12 USD) at my local convenience store. while trying to get it to work i ran into a few problems, so i thought i'd share my solutions in case anyone else has a similar problem (the cards are not very intuitive to use if you can't read chinese, and i found the voice menus to be pretty difficult to understand).
1.) buying the card: despite the fact that most countries use scratch-off paper or plastic cards to give you the phone recharge code, and indeed many stores in china have plastic china mobile recharge cards in their display, don't panic if you get a blue-and-white piece of paper with holes down the sides. this is also a recharge 'card'.
2.) reading the card: everything in the instructions is in chinese, and to add to the fun there are several strings of numbers on the inside of the recharge card. don't panic. your 'recharge pin' is the long number beginning with '09' (or it may start with a different number, such as '19'). this is all you need to know.
3.) using the card: dial 13800138000 (yes that's an extra '0' at the end) from your mobile phone, then dial 2, then 2, then 1#, then 1, and then enter your recharge pin followed by the # sign.
4.) you should hear 'please wait a moment', then a success message. that's it! your card is now recharged.
if you need additional help: as difficult as the recharge cards are to use for people who don't speak chinese, the chinese/english bilingual china mobile customer service operators are VERY helpful. dial 11086 (note: this number used to be 1860. if the new 11086 number doesn't work for you, please let me know!) from your phone for assistance -- if you get an unhelpful operator or you can't get through, just hang up and call again, and you should get promptly connected to another (and hopefully better) operator.
thanks to the helpful reader David who also added that the number to dial if you want to check your prepaid China Mobile balance is 10086.
hope this helps someone else trying to do the same thing!