Products are ordered from MOO's site, although SmugMug has included a link to Moo for business card printing, from the Buy button in its galleries. Creating a photo product is as simple as selecting the product type, selecting SmugMug as the source, authorizing access to your public photos on SmugMug, and selecting the photo(s) you want to create products from. The sole limitation is that currently, MOO can't access photos which have been password protected on SmugMug.
For more details, read the MOO blog, and visit the SmugMug page on MOO's site.