Slack Connector add-on can be found on Atlassian Marketplace and installed directly from your JIRA Cloud instance using Universal Plugin Manager. Once installed, main Slack Connector configuration can be accessed directly from UPM by clicking "Configure" or "Get started" button. Please check out Configuration page for more details on configuration.
The first step after installing add-on should be connecting your team's Slack instance with JIRA. See Connect with Slack page to get information regarding this process.