It used to be that one would install dump1090-mutability for ultimate flexibility. Then there was the adsb-receiver project which installed multiple feeders, all scripted for you. Both projects have fizzled, and FlightAware's PiAware image has done a great job of continuing on with the mutability build of dump1090. It is by far the easiest installation to do.
MLAT is set up in Radarbox by installing mlat-client from the radarbox repo that is added when you installed the feeder. You will also need to add your coordinates via the website station page, or by /etc/rbfeeder.ini. The format is:
lat=xx.xxxxxx lon=xx.xxxxxx
Now, the quirk with the mlat-client is that the rbfeeder app does not hook into it when using systemctl to restart. It will however start up correctly with a reboot. This can probably be fixed in the systemd file, so that the mlat-client starts up with rbfeeder correctly. Otherwise, you will not have MLAT. Once set up correctly, look at your station page, and it will show if MLAT is enabled.