Hi Gerry, I am happy to answer any questions you have about working with the BoldGrid Backup plugin.
The only way to use your local machine as a directory for backup storage is to setup a local FTP server and supply that connection information while configuring your FTP/SFTP settings.
The process for setting up a local FTP server varies depending on which operating system you are using and there are a number of guides available for Windows, Mac, and Linux that will show you what you need to do.
However, I do not recommend using your local machine as an FTP server since doing so will open up your home computer to a number of attack vectors and could lead to a compromised system. If you need a different location than your Web Server for storing backups then I suggest using either Google Drive or Amazon S3. A free Google Drive account has up to 15G of storage and will be much safer than using your home computer for backup storage.
I hope that this information helps you choose the storage solution that best fits your needs and please let us know if there is anything else that we can do to assist you.