This works well for production bots, but requires some configuration first. For testing purposes there is an easier way to create a Slack app with a bot user and to add this to a Slack team.
From the ‘Deploy’ page in the Watson Conversation tool you can choose to test your workspace(s) in Slack.
Next you can choose your Slack team and authorize the connection via OAuth. This creates a Slack app and bot user.
To interact with Watson you can invite the bot user.
Note that there are currently some limitations and at this point this functionality is only for testing purposes.