You'll find a link to your live ads in two different places. First, if your listing was auto-boosted, you received an email from Boost letting you know your ad is about to go live with a link to see your ad:
You can also find this link in your dashboard. Tap on "View" next to the ad you want to see:
On this page, you'll see a link below your ad preview to view on Facebook and Instagram. Click on the link to see your live ad.