Open the Cursor page, paste an API key from your Cursor dashboard and save it. Cursor's agents then show up in the model picker, labelled as agents because they work differently from chat models.
Pick one in a branch and include the GitHub repository in your message, like "in github.com/you/app, fix the failing login test". The agent works on that repository and reports back into your conversation. In Explore, a Cursor variant tells you before it starts if no repository was given.