Hummingbird - Can't activate Uptime Monitoring

I recently installed the Hummingbird plugin to speed up my site. Everything seems to be working fine, but when I tried to activate the Uptime Monitoring part through my WP dashboard, nothing happened. I eventually found that I could activate it through my WPMU DEV Hub.

When I try to activate it through my WPMU DEV Hub, I get the following message:

" Your domain does not appear to be publicly accessible on the Internet."

I'm not sure what this means or how to proceed, as no additional information or suggestion is provided, but it sounds really scary!?

What does this mean and what do I need to do to enable uptime monitoring?