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