For this go back into your MobFox account and open the Sites & Apps page.
Locate the app you want to add the backfill to and click on the little BackFill label.
Next, activate the backfill for this app.
Select InMobi as the backfill option.
Enter your app ID inside the filed for InMobi Site ID.
Click on Save so the option will be activated.
What just happened?
You have set the InMobi backfill option in your MobFox app settings. For iOS, the process is very similar; you just need to create an iOS app in InMobi.