The loop number setup is a little tricky. Turn on the debug counters; to do this go into the events to the Debug -> Debug Variables group, right click the group itself and click Edit, then make sure "Active when frame starts" is checked off.
Now when you run the frame, you'll see some extra information. Second counter from the top is the current music position. This is the counter you want to make a note of. When the music reaches the point when you want the looped segment to start, pause the game and write down this number. Now, unpause. When the music gets to the end of the looped segment (that is, the very moment it's supposed to loop back), pause again and write down that number.
Those are the Loop_Start and Loop_End values, or close enough approximations. Fill them in and play. If it sounds a bit off, you can go back and modify the numbers until you get it juuust right.
If you enter in zeroes, the track won't loop and will just play once from the start of the file to the end of the file.