The Slack integration uses the ProjectLocker WebHooks notification feature, available on the Venture and larger paid plans.
To get started, first go into your Slack channel and click the name of the channel at the top to get the channel menu, and click Add A Service Integration. Then, choose Incoming WebHooks. Choose your channel from the dropdown and click the button to add the WebHook. Now, grab the URL from the section "Your Unique Webhook URL".
Now, tell ProjectLocker to point at your Slack Webhook URL. Just login to ProjectLocker Portal
and click Projects at the top. Then, click the name of your project to go to the project's management page. Then, click the link for Manage Webhooks on the left. Paste in the Slack Webhook URL and save.
After following these steps, your ProjectLocker project is now linked to your Slack channel, and commit notifications will show up for that project in the channel you specified. You can repeat this process to link other projects to the same (or other) Slack channels to enable notifications for all of your projects.