Where to Upload Files for an Addon Domain
Each Addon Domain is assigned to a directory on your account. This directory is typically a subdirectory in your public_html.
Uploading Your Site
After you have assigned an Addon Domain via the Bluehost domain manager, there will be a sub-folder created in your "public_html" folder (e.g. "/public_html/addon_domain_foldername"). You will need to upload the files for this Addon Domain to the subfolder created.
Note: If you are unsure which directory an addon domain is associated with, click on the "Domain Manager" tab at the top of your cPanel. Next select the domain name from the list. Now look to the right and under the "main" tab you should see "Addon Directory: public_html/addon_domain_foldername".