I assume these are URLs for movies not already in your database. I'm not certain, but I think the import plugin would need at least a Title in this case—in order to create a record in which to save the data (in this case, the URL). If you don't have titles, just add a column of arbitrary names (e.g., URL-1, URL-2, etc.). Don't forget to configure the plugin to import Title and URL. Once imported, set the IMDb plugin to overwrite Title. When it's run on the records created by the import, it will replace these dummy titles.
[Edit] You beat me to it. I'll leave this here in case it helps anyone else understand how the import works.