One of the first things you'll need to do after creating a loop is add documents. You will have the option to add documents from your templates, computer, or by email. Once you add a document to your loop it cannot be deleted, only archived.
If you're practicing with a test loop, try uploading a document using each method.
If you are using a loop template, you may see some documents are already in the Documents section. In that case, click the Add Document link to the right of the folder. The 3 options for adding documents will appear.