Hm... can you take a look at my capx?
Its pretty straightforward but i dont get result i was looking for...
Your .capx works ok for me.
I added a Textbox(textarea) and set to Json.GetValue to show returned json data.
I also added a normal text object and set text to Global A.
All seems fine.
What browser / error messages are you getting?
BTW using C2 r255, Chrome and latest json++ 1.07.