See: File > Examples > Topics > Advanced Data > LoadSaveJSON

Or: http://processing.org/reference/loadJSONObject_.html

Thanks

however, now im using Processing 1.5.1.. how can I use this in Processing 1.5.1? 

You can't. The full data api including JSON support was added with the 2.0 release.

If you are using 1.5.1 then you have to download a contributed library and use that instead.

There are several of these, see Google and the forum.

If you want something really quick and simple then  http://www.mkyong.com/java/json-simple-example-read-and-write-json/   may help you.