When a real estate office or agent invites you to be an advertiser in their app, you'll receive an email invite with a link to learn more and activate your ad.
In the email invite, click the link that says Learn More. You'll then be taken to a more info page.
From the more info page, select the Activate My Ad button if you're ready to purchase.
Fill out the form, and select Purchase. You'll then be able to create your ad profile and upload your images.