Once you've installed Addition there are 3 key things you should do now to get Australia Post retail rates displaying correctly in your store.
Setup your Shipping Zones and enable Australia Post rates.
If you've already setup your store but are having problems, check you haven't accidentally missed any of these important steps.
If you have an Australia Post business account with discounted rates, such as MyPost Business or eParcel, and you plan to use the app for order fulfillment, now's a good time to also connect up your business account, too. Follow these guides: