On the DP-24, I connect to my laptop via USB, which makes the DP-24 appear as an external storage device. I navigate to the Music folder, and inside are folders for each song on the card. Select a song folder, and you'll find the last saved mix (whether mastered or not) at the top of the file list, with the song name and the WAV (wave file) extension, which can be copied to the laptop like any other file. The other files in the song folder are the individual tracks, however the naming convention makes it a trial by error method of finding which track is which.
Alternatively, you can simply remove the SD card, insert it into a card reader on a computer, and again, simply navigate the files as you would with any other external device, like a thumb drive. The current mix will be the last Save you performed on the song, and will be located at the top of the list as (songname).WAV.