Install your iZotope plugin from the Splice desktop client.
To get your iZotope plugin up and running in Ableton you need to check Ableton's AU/VST settings.
Navigate to :
File and Folder
Make sure "Use AudioUnits/VST" are all checked and hit Rescan.
Windows users will only have the option "VST Plugin Custom Folder", manually navigate to the location you installed your iZotope plugin.
Neutron 3/Ozone 9
Neutron 2 / Ozone 8