I would like to use a time capsule back-up to set up a new mac but migration cant find the file?

I would like to use a time capsule back-up file from an old macbook which I no longer have.. to set up a new macbook . The back up folder is stored on an eternal hard drive. When I run Migration assistant on the new macbook, it prompts me to "select the drive" (of where the back up is found) but the drive does not appear. How do I tell migration assistant where to find the the hard drive let alone the folder/file of the back up? The external harddrive is connected by USB and is visible on the desktop. thanks.