When sending json, I usually capture a few successful requests in Wireshark without tsung running.
Then I find the IP address of one of the packets select the 'Analyze' menu and select 'Follow TCP Stream'.
This allows me to see the actual data sent.
The data can look something like this:
data.text=U.S.+intelligence+agents+have+been+hacking+computer+networks%0A++++++++++++++++++++++++around+the+world+for+years%2C+apparently+targeting+fat+data%0A++++++++++++++++++++++++pipes+that+push+immense+amounts+of+data+around+the+Internet%2C%0A++++++++++++++++++++++++NSA+leaker+Edward+Snowden+t&type=text&data.sender-id=8a5b1c2f-0589-464c-82c4-b8f4e280511a'
Then I modify the tsung xml to look like:
content_type='application/x-www-form-urlencoded' contents=''
And paste the data in the contents.
You could also try escaping the quotes using " but I never had any luck.