Test data downloads from your local machine in AppData\Sawtooth Software, Inc.\SSIWEB\LocalWeb\htdocs\studyname. This will always work unless your local apache is not running.
Field download data allows you to enter the link to your admin module url and SSI Web will download any data you have collected from the field and put it into SSI Web
If you are getting a 400 error that means your url is incorrect.