After you log in to fireRun.io, you will find the "Add to Slack" button in the Top Resources section on the main page.
This will bring up a window with brief instructions. Click the "Let's go" button.
You will be prompted by Slack to give fireRun.io permissions to post to your Slack channel. Click the dropdown near the bottom of the page to select the channel you want. Then click the "Allow" button.
You will see the "Slack Integration Successful" message. Then you can close the tab.
In fireRun.io, you will now see the Slack activation button showing as Active.