Install your iZotope plugin from the Splice desktop client. If you choose a custom installation destination, take note of the file path.
Once installed navigate to FL Studio go to:
Options
General
Manage Plugins
On the left, you can add plugin search paths, add the location you installed your iZotope plugin into and hit Start scan.
Neoverb
RX 10 Standard
Stutter Edit 2
Nectar 3
RX 10
Vocalsynth 2
Neutron 4 / Ozone 10
Ozone 10
Neutron 4