Hi Tod, thank you for your question!
If you want to use your local machine as a backup storage location then you will need to set your computer up to function as an FTP server. I found this guide that shows how to do that on a Mac. Once you get the FTP configuration finished you should be able to supply the credentials using the FTP option in the Backup Storage settings.
I hope this helps you work with your backups as desired and please let us know if there is anything else that we can do to assist you.