Go to your admin panel. From there you should see Shop Administration as one of the blocks on the left toolbar.
Shop Administration - General Settings to edit amount of credits per post.
Shop Administration - Add new Item to add new items.
Note that if you code yourself a new item, you need to add it to simplemachinesroot/sources/shop/items/ in order to add it from the Add New Item Page.
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If you don't see the Shop Administration section of the sidebar, then my guess is SMFshop didn't install properly. Try installing it again, and if that doesn't work, the only thing left to do is bug Daniel.