What is the device that is sending the data?
There are two ways to send a datapoint to device cloud.
The first is just with a POST and the second is by inserting it into the EDP stream of the cloud connector. The first one is very simple and you can see how to do this if you go to the API Explorer under the Documentation tab in device cloud. It will allow you to create a data point and export it in python.
The second requires a device with the cloud connector already running. If it is a WR21 then you should use the DIA framework and allow the data to be transmitted within the EDP protocol. This is a little more complex but still fairly simple if you use the Digi ESP for python. You can get a copy of it here: http://www.digi.com/support/productdetail?pid=5499&type=drivers
The benefit of using the second method is that it is more secure and is using the same channel as the device monitoring.