You have to use the Streaming Client Software on the Exstreamer and check if "playlist.m3u" is set in the 3rd URL.
Copy your mp3-files into the root directory of the USB-stick.
After rebooting, the Exstreamer will find the attached USB-stick and play the music if there is no network available.

The Exstreamer with default settings will auto-create a playlist and use it.
If you want your own playlist with your playlist with your own song sequence, then create an own playlist.m3u file
and copy this also into the root of the USB-stick.

To test it you simply have to copy/configure the "playlist.m3u" entry into URL1 in the setup
or you remove the network cable, in this case it will also play the USB audio even when the "playlist.m3u" entry is in URL3.

Note: There are some rules for the USB-stick. See USB_device_compatibility_list#Limitations