To work, the Vimeo import tool needs you to provide it with a Vimeo access token. This access token is used to retrieve the list of your videos, and to access the video source files.
The generation of an access token is done in two quick steps:
To create an application, go to your Vimeo applications list, and click on Create an app
Once your app has been created, you'll be redirected to the application settings page.
From here, you'll be able to generate a new access token.
In the Generate an access token section, select Authenticated (you), check the Private and Video Files check boxes, and then click on Generate.
Just bellow, a new item will appear with the Token value (8011xxxx...xxx in the screenshoot above), this is the one you'll have to copy/paste in the import tool.