To add a new Deployment Key to Bitbucket, navigate firstly to your repository's Settings via the link near the bottom of the sidebar, then to the Access Keys page:
Then click the Add Key button:
How give your new key a description, paste the key in, and click Add Key to finish.
You'll now just need to head back to your repository's Overview, then copy the SSH URL to the right hand side of that option:
You can now see further information about how to update the repository configuration in Deploy in this article.