If you cannot find your purchased/activated Let's Sing content, you can proceed as follows.
Note:
Make sure that the code/voucher for the "Free Songs Pack" has been activated on the profile you want to use to access the content.
1. Either in the main menu or in your game library press the option button while hovering over the application
2. Here you will find the "Manage Game Content" Button
3. Now you will see your DLCs and you can download any missing content.