It is simple, you can sign a petition. But that is the simplest thing, and how effective it is, is doubtful. But you can do something else, something much more direct, and it is not difficult. The first proposal is how to make the Chinese embassies a piece of art.
1) if you have a phone
Here is the list of Chinese embassies you can use, find out the phone number of the one in your town, and give them a call, tell them that you have a friend in China missing and his name is Ai Weiwei, he disappeared on the 3rd of April, and you need their help.
If you are not a Chinese speaker, and you want to learn a bit of chinese:
Ni hao ，wo xu yao ni de bang zhu 。wo you yi ge peng you zai zhong guo shi zhong le 。 。ta de ming zi jiao ai wei wei 。
Hello, I need your help. I have a friend disappeared in China, his name is Aiweiwei.
2) If you happen to pass a Chinese Embassy, please print out this Notice written by Aiweiwei’s mum and sister looking for him, and give it to someone in the Embassy. Or if you can post on their walls, toilet, etc.
releaseaiweiwei 
That should be “shi zong le’, not zhong. Also you need to give a couple of pronunciation hints – ‘x’ is a quiet ‘sh’ sound; ‘z’ is ‘dz’. Otherwise it’s good