Unfortunately the systems hosting the uploader appear to be offline. Rest assured, we're looking into it!
All stacks already online are unaffected: they are hosted by the Internet Archive and you can continue to view and run them at https://archive.org/details/hypercardstacks
We hope to have the uploader back online soon. Meanwhile, if you have stack you want to upload, please feel free to email us at HyperCardOnline@gmail.com
Thank you for your patience!