What directory on my computer are stock-asset downloads saved to?

The files are stored in the application's data directory which can be accessed in the Lightform Creator preferences. If you go to preferences --> support --> data directory, they are saved in a subfolder called "Storyblocks", however, the files are stored using a naming format that is optimized for use within Creator. In order to see the files by name, you'll need to search that folder for .mp4 files.