Spotdox requires that the Spotdox folder is synced to your computer. Selective Sync is a Dropbox feature where you can tell Dropbox to only sync certain folders. If you use Selective Sync, make sure that the spotdox folder, which is in the 'Apps' folder is synced. The Apps folder is usually called 'Apps' and is usually located at the root of your Dropbox. It may have a different name, especially in different countries, where a localized equivalent is used.