So you've added a client's Hubdoc account, but you can't find your client's documents. You will have to toggle into their account before you can start viewing uploading documents.
To toggle into a client's account:
1) Ensure you have been added as a collaborator on their Hubdoc account.
2) Click on the three bars (also known as the hamburger stack icon).
3) Select the client's Hubdoc account or click "My Account" to toggle into the Organizations Page.