after you have import the music, go to the sound editor ssearch the music and right klick on it, then choose use as music and now u can use it. use a trigger to play for example:
Event: every ?? seconds of gametime
(type in a number. the best is if the song are 1minute long then type in 60 so after the song finishes it will begin again)
condition: none
Action: play music (Select the song u want to play)