Before using Threat Exchange, you will need to have a business verified Facebook app.
Please see the Terms and Conditions—depending on your company, you may wish to obtain sign off from your company's legal team.
If you haven't already done so, check that you have changed the app's status to "Live".
The name ThreatExchange appears with a green checkmark, and you see the subproducts "Descriptors", "Tags", and so on, in addition to "App Review" which you had seen up until now.
Do a power-search for tag "testing"—you should see results.
Welcome to ThreatExchange!
Next, learn about Best Practices for Using ThreatExchange