To use SmartPost, your items must already be in your Listings. Go to your Inventory page and select your items.
Click List Without Posting to Facebook.
Then go to your Listings, under the SmartPost tab.
Then select a Facebook group, and it will automatch your already created albums you would like your Listings to post to. It will show a percentage of confidence in the automatch. Double check it has selected the appropriate albums.
If you do not see your albums have been automatched, click the Do Not Post field. A selection of all available albums in that group should appear, or you can Create New.
There is a great tutorial video here: