For more details on the Slack app, see our main article: The Trello app for Slack
To connect the Trello app for Slack to a Trello Workspace, go to your Workspace settings page. Only Workspace admins will be able to connect the Slack app.
A Trello Workspace is required to connect the Trello app for Slack. For more details, see What are Workspaces?
What if I already have the old app?
The old app (Trello Alerts) allows you to receive updates in Slack to changes to Trello boards and cards. This is the app: https://trello.slack.com/apps/A0F814C4R-trello
We are working on bringing this functionality to the new app.
In the meantime you can continue to use the old to receive updates in Slack for Trello changes.
Once you have added the new app you can go to your Workspace settings page and click the "Configure sending changes to boards and cards to Slack" link.