Hi Paul,
Welcome to the community!
So this isn't well documented, I'll see that we fix that, but to use a custom catalog with Slice Machine you need to add the field to you Slice in your model.json file directly in your project, i.e.:
        "catalog": {
          "type": "IntegrationFields",
          "config": {
            "catalog": "sephora-demo--product-catalog-sfcc",
            "label": "catalog"
          }
        },
Then, you need to make a dummy change in the Slice Machine UI to be able to push the changes to your repo.
Let me know if you have any questions.
Thanks.