Construct 2 doesn't accept just any JSON string, but wants it in a special format
something like this, if I'm not mistaken
and so on
If you have the full version of Construct 2, you can start your layout in debug mode and check out if the keys are filled with your json string's data - I think they aren't.
Also note that on created does not activate for the dictionary if you don't create one via some event. If you don't create it, use "on start of layout" instead to add all the relevant keys to the dictionary from the json string.
@LittleStain: I think Dictionary is the right choice, since you have name -> value pairs. In an array, you would need to do a for each x and then first look up the correct row that contains the key, before you can retreive the value for it.