Sorry for the frustrations. Unfortunately FTP was taken away do to security issues. However you should be able to upload multiple files using the Sitebuilder Plus editor. When you login to your account, click Websites Manager on the left. When the page loads, click the arrow next to Edit Site and choose "Edit with Sitebuilder Plus".
When the online editor opens, click your domain or sitename at the far top and choose "view file manager. When your file manager opens up, click the import button. You should be able to select multiple files by holding down the Ctrl or Shift key on your computer.