You didn't put the card into the Zenpad while it was turned on, did you? Try restarting it. Or you can run an app like
media.Re.Scan. (Media apps in Android don't fine the actual media files, they look in a database created by Media Scan every time you turn the phone on or restart it - media.Re.Scan forces it to rebuild the database. If you have a few thousand songs on the Zenpad [and with 400GB SD cards available, you could easily have that many], it would take each app many, many minutes to scan the whole card every time you ran a music app, a movie app, etc. Asking the database for all music files" takes almost no time, and you have your list of music, whether it's 5 songs or 5,000 songs.)