Access to the design kits from within the app (File > Get UI Kits) takes you to a 404 page.
Thanks for your feedback. This is something we're aware of and are looking to fix. In the meantime, you can look up UI kits here: https://www.behance.net/search/projects?search=xd+ui+kit&tracking_source=typeahead_search_suggestion
Hope that helps!