If you're using Cubase to create your tracks, just follow the directions below to export them as 320 kbps MP3 files.
Once your tracks are finished, be sure your track locators are set to the right length. A shortcut for this is to click on a track, then click P on your keyboard. Once that's done, click File in the top left corner, then click Export > Audio Mixdown. When you do that, the Audio Mixdown window will appear.
In the Audio Mixdown window, you'll be able to name your file, choose where to save it, and choose the file format. To export as an MP3, choose "MPEG 1 Layer 3 File." Under the file format menu, you'll be able to set your bitrate to 320. When that's done, click Export.
Once it's done exporting to your device, you'll be all set to upload your track(s) to Digital DJ Pool :)