Hi! I bought RSI 1. In Native Access, I only see ADD LIBRARY. I don't understand where to download it.
Sounds like you're still on Native Access 1, which is legacy and no longer maintained. It won't reliably show newer products and updates anymore.
Download and install Native Access 2 from here:
Once installed and logged in, you should see RSI 1.
Have you read the manual which you can obtain from the product page of RSI 1—scroll down that page to the "At a glance section"?
https://www.native-instruments.com/en/products/nks-partners/10-phantom-rooms/rsi-1/