Note that until you publish the Opt-In Data Collection content unit in the Marketing Dashboard, you will not be able to download the CSV file. If you just "save" the content unit so that only test devices can see it, attempting to download the CSV file will result in the following error:
"error": "500 Internal Server Error"
This is true even if you populate the Opt-In Data Collection with data from test devices. If you want to test the populating and downloading of Opt-In Data Collection content units, we recommend creating placements that don't exist in the retail version of your game (e.g. "opt_in_data_test_point") and put those placements in unreleased test builds of your game. That way, you can "publish" and test the Opt-In Data Collection content unit without exposing test versions of it to your actual users.