Thank you for letting me know about this!
If you go into the options menu in game, under "General" there is a button to open the "main" assets folder under your Documents (Documents\TabletopClub\assets), it should automatically open file explorer when you click it - try placing your asset pack folder in there and either clicking the "Reimport Assets" button or restarting the game.
If that doesn't get your cards to appear, then it would be much appreciated if you submit a bug report on GitHub so I can try to help you further and track the bug for future reference :)