To add a single item of audio to the Media Finder, click on the + icon.
This will then open the media finders item card,
From here you can choose to add audio using the folder + icon on the top left-hand corner of the window.
If you are wanting to record directly into your blank UID, you can instead click the microphone icon and do this.
Remember to assign your piece a UID and a TYPE ahead of trying to save.