The path is public_html/addondomain.com and can be accessed via File Manager or FTP. Please remember to replace addondomain.com with the actual name.
Any easy way to access this folder is to login to cPanel and click the Addon Domains icon. Now click the link under Document Root. This will take you to the correct folder in File Manager.
If you still need help understanding, feel welcome to contact HostMop via live chat or email support@hostmop.com.