Welcome to the Reaper Forums!
In case you haven't already found them, make sure to check out the User Guide (
https://www.reaper.fm/userguide.php) and Videos section (
https://www.reaper.fm/videos.php) in general.
If the info in these links doesn't solve the problem, please post more details about your setup, like:
- Windows or macOS?
- How is the Alesis Nitro Kit connected?
- What isn't working, exactly?
- Which steps and/or settings have you already tried?
- etc..