Hi there,
AtoM should include the path to the digital object in the digitalObjectURI column of the CSV file on export. In most cases, you can take this URL, remove the base URL at the start (e.g. the homepage URL of your AtoM site), and replace it with the default installation location of AtoM to get the absolute path - if you have followed our recommended installation instructions, this is typically /usr/share/nginx/atom.
I've previously explained this with some further examples in this older thread:
Hopefully this will give you the information you need!
Cheers,