Do you try to use the Calibre OPDS connection to connect to it from within the Freda app on the same machine? I tried the loopback local address (127.0.0.1:8080), the local IP address, etc, on the local machine. I could get a browser to connect to Calibre locally and load up a reading view of my ebook library, but Freda couldn't connect locally even if I opened up for the Calibre main app and the web server executables in my firewall.

Or, do you browse within Calibre itself, open the containing folder, and click on the book to open it with Freda? I can make that work. I don't see a way to switch the default epub viewer within Calibre to something outside of Calibre's own viewer.