You've completed it to some extent, but it would be frustrating without a BGM, right? If you look at the Editing Window in the center, you will see a sound icon with a "none" button indicated. That button is the one that can change the BGM.
Click this button and select the BGM music you want. If you hover your mouse over any music for a while, it will automatically play a preview. Once you've chosen the desired BGM, click confirm.
Now you can see that the display has changed from "None" to the selected music! Shall we take a listen? Just press Space
on the keyboard or click on Preview at the top of the screen! You'll know the current selected slide is playing along with the music. If you want to view the entire sequence, click the ▼ button on the right side of Preview and select Preview.