Installation for Chatbot is the same as for any other extension.
Until Chatbot becomes approved for automated download, you will need to manually download it and extract it to your extensions directory.
You can find the latest stable release here: https://github.com/3sd/civicrm-chatbot/releases.
Once you have downloaded and extracted the extension to your extensions directory, it should appear on the manage extensions page. Enable it and you should be ready to get started.