You must punch a hole in your firewall otherwise your machine will not be backed up. You can configure this exception just for Retrospect:
The easiest way to do this is to open a command-line or PowerShell window and type the following two commands:
If you look at your firewall configuration, you should now see two exceptions named "Retrospect TCP" and "Retrospect UDP"
Using the command-line, simply type the following two commands:
You will see "Retrospect TCP" and "Retrospect UDP" in your firewall exceptions list with a check box next to each.