I stumbled across this topic when looking for something else, but in my experience, the photos already need to be in your Smugmug Library if you want to "sync" in reverse like this. Please correct me if I'm wrong as this is my only major complaint with the SmugMug plugin.

For instance, I share a SmugMug account with another user to catalog photos for a high school athletic team. If I want to sync her photos into my LR catalog, I actually have to manually download the photos in a web browser, import them into LR, and then sync the hierarchy and the photos. If they aren't already in my library, I'll get zero photos as mentioned in the post above.

This probably makes sense because you can't really have photos in the Smugmug Publish tab if they don't exist in LR right? Ideally in this situation, there would be an option to create a folder to bring all these unmatched/uncataloged photos into your library, but I don't think this is possible. This is a major stumbling block in a sharing situation like mine or if someone has a sizable SmugMug presence that was not previously tied to LR. The workaround in either case is to manually download all the galleries and import them into LR prior to trying to sync. You can import them into existing folders based on the individual Smugmug galleries or one big folder called "SmugMug Imports" and then organize later.

One word of caution about importing photos and then syncing as mentioned above: DO NOT change anything in the photos prior to syncing with Smugmug! Even a slight change in name or metadata will likely confuse SmugMug to the point where it will ask you to resolve conflicts while syncing. Generally, if I don't change anything I don't get any conflicts, but a minor change usually leads to a lot of tedious conflict resolution. This wouldn't even be an issue if you could just import the photos automatically via the plug in.

Other than that, the plugin's integration between SmugMug and LR is a godsend.