Go to Main Menu > Plugin Marketplace in Mattermost.
Search for "Bitbucket" or find the plugin from the list.
When the plugin has downloaded and been installed, select Configure.
If your server doesn't have access to the internet, you can download the latest plugin binary release and upload it to your server via System Console > Plugin Management. The releases on this page are the same used by the Marketplace. To learn more about how to upload a plugin, see the documentation.