Hi Miroslav,
In Mendix 11, you can use the JSON library that is built into Mendix under the package:
com.mendix.thirdparty.org.json
As far as I know the root org.json package is only available automatically if one of the modules in your project (for example, a marketplace module) already includes it. Otherwise, it won’t be accessible.
Alternatively, you can still download the original json.jar and add it to your project’s userlib, then configure the build path for it in Eclipse.
I hope this helps.