I had the same issue. Here's what I did to do to make it work:
Changed the name of the app to TA-slack_alerts (probably not necessary)
Added empty tags.conf and eventtypes.conf files to TA-slack_alerts/default
Modified slack.html to use an input instead of a text area due to getting an error stating that there was no message specified even though there was. This was in TA-slack_alerts/default/data/ui/alerts
Now it shows up and I'm successfully able to have it send me alerts when my correlation searches fire.
Hope this helps,
-Dan
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