Downloading a dataset from BigML is very easy. If you are using the BigML API, please read this subsection of the documentation to learn how to retrieve a dataset. If you prefer the BigML Dashboard, please go to the dataset view, then click the 1-click action menu, and select DOWNLOAD CSV:
Note: if the dataset you want to download is in the BigML Gallery, you should clone it first, and once it is ready in your Dashboard, you will be able to download it following the steps explained above.