How Do I Use My Facebook Business Page?
by Kim Snider - Get free updates of new posts [thrive_2step id='749']here[/thrive_2step]
Ad accounts are attached to you. So are Business pages. So when you connect, you will be connecting to your ad account, which includes all profiles and pages. When you go into Ad Manager to create the ad, that is where you specify which page the ad is tied to and what custom audiences to include/exclude, etc.
So, to put it another way, you designate the page at the individual ad level, not the ad account level.