You can locate your ad account ID by logging into your personal account and navigating to your Ads Manager
). You will see that your URL adds a "act=[there will be a number here]" to the end of the link in your URL pane. You'll want to write the number you see here.
For example, if my URL is https://www.facebook.com/ads/manager/accounts/?act=1234567, then my account ID would be "1234567."