Hey, Adisuthar.
It appears that you're not finding the iHeartRadio application for Apple TV in the Apple TV App Store. I'm happy to help.
iHeartRadio -- Free Music & Radio on the App Store -- Going by this application preview, it is showing available in the Apple TV App Store. Try searching for the application again.
How to restart your Apple TV -- If the application still isn't showing in the Apple TV App Store, restart your Apple TV and test. If it still doesn't appear, contact the developers for further help using the "Contact Us" button near the top-right -- iHeartRadio Help
Let me know how that goes.
Take care!