You NEED to download the FileHub application. Get it HERE.
Find the level that you want to download and click the button.
Click the 'Open In' button. It will open in File Hub.
Click the the new file in your list. If it asks 'Do you want to unarchive it?', answer Yes. NOTE: After you are done with the files deleting them from FileHub will make it easier to find the next level you download)
Click the the extracted file(The one with a ?) At the bottom of the pop-up menu it should say 'Open In' select that!
Scroll to the right until 'Copy to Happy Wheels' (Hint: You're almost done!!!)
Now, for the final step, you can either Import to play later. Or you can Import and play now.