From the Menu, click Change Listing (or My Incomplete should the listing be in this status), click the appropriate listing, then Edit. Under the Documents tab on the right-hand side of the page, click the chevron and select the document to be uploaded.
This is extremely important especially for private documents such as the Listing Agreement, Withdrawals, Price Changes, Extensions, etc., as the system will understand that they are private by choosing from the drop-down. In the case of a Listing Agreement, it will also trigger the system to approve your submitted listing.
If you wish to upload a public document and the option is not available within the drop-down list you may type the name of the document into the Description box prior to uploading.