If you're using Safari, downloading data as a CSV from Beacon can sometimes cause a problem. Rather than downloading the CSV file, sometimes Safari will open up the CSV file inside Safari, which looks something like this:
Pretty scary, right? Luckily, there's a simple solution. You can save the scary looking page by clicking File > Save As.
When prompted, make sure you set the Format to Page Source.
After saving, the file will have been correctly saved to your PC / Mac.