WHERE IS IT?
In Google Drive, you get the file id from the link provided to you when you publically share the file. You must ensure the file is set to "Anyone with the link can view."
WHAT TO DO?
Copy and paste the link you get from Google into the external resource field. You will usually be pasting these into one of the audio slots or one of the image form fields when building a scene. Imp Director will restructure the pasted link automatically so that you can use the resource outside of Google Drive and so that it works with your ALEXA device.