There might be only a few reasons why an ad account may be missing from the integrations.
Your Facebook profile doesn't have the necessary permissions - make sure you are an admin of your Business Manager and the ad account you're trying to integrate;
It's just a delay after you added this ad account to your BM or got the admin permissions;
Facebook's glitch which can be fixed by reconnecting Facebook profile in integrations, adding and removing the ad account from your Business Manager, or toggling off and on the admin permissions for the ad account in Business settings:
1. Try reconnecting your Facebook profile using this link: https://revealbot.com/auth/facebook?auth_type=reauthenticate
2. Remove the ad account from the Business Manager and add it back, try reconnection again, or
3. Toggle off and on the admin permissions for the ad account in Business settings:
If the ad account is still hidden, don't hesitate to contact our support team to help you reveal it!