Seeding via URL is basically seeding via a USB-attached drive on a Windows computer. The URL should point to the drive letter of the USB drive on your Windows computer. I.E. – if the USB drive that contains the seed(s) has drive letter G:, the URL for the seed import should point directly to the *.gz file and not a specific folder within G:
Attached is a screenshot of the import options when clicking on the seed icon on the Vault.
You can use IIS or something like this to provide the temporary URL: http://www.rejetto.com/hfs/.