In a single SMTP server case, you should have at least 2 mailboxes - 1 for collecting bounces and the other for collecting spam complaints. Bounce mailbox will be your return path email itself. Abuse address will be something like abuse/postmaster@example.com - where example.com is your sending domain. When you create a delivery server of type SMTP in MW, you will also be creating a bounce server and attaching that to the delivery server. Here, the "Email" you specify on bounce server settings will be your return path, if you keep it empty, return path will default to From address on your emails. Whatever so, your return path will be receiving the bounces back.Because you already added the bounce server along with delivery server, it will process your bounces automatically. For FBL, you have to register this email address with different ISPs to receive abuse emails on it. You have to add/create this mailbox on MW to process the complaints now.
I presume you have a basic understanding of how an email system works, the above instructions should be enough for you then.