First, go here http://download.qt.io/online/qt5/ to figure out the url to the version of repositories you need. For me it's linux 64 bit, so I'd click down into that path until I got to http://download.qt.io/online/qt5/linux/x64/online_repository/.
Copy this url into memory.
Then, in the repositories tab of the Qt maintenance tool, select "user defined repositories" (see image in the previous post) then to the right of where you have just clicked, paste in the url you just copied. It's difficult to tell there's even a field there, it's almost hidden. When you hit the right spot the url will paste in and you should be good to go.