Go to Smart Search & Instant Search admin panel → Integrations page, and turn the toggle on. Next, click Apply Changes.
After the changes are applied, you need to click Force re-indexation. The indexation takes more time than usual with Infinite Options integration enabled.
Note: You should run forced re-indexation each time you change custom options sets. Wait about 5 minutes after the changes are saved, go to Smart Search & Instant Search dashboard and start indexation.
After the indexation is complete, you will see the filters by Infinite Options custom sets in the Search Results widget.