I don't think you can download the default sound bank for the Roland D50, however if you conduct a search on Google for the D50 sound banks you'll find plenty websites with patchsets, additional sound banks, manuals, and other information that might be relevant to you.
Access the following link that was created especially to filter other results and display only what is relevant for you. You can then use D50 Librarian to manage your soundbanks and library.