first of all make sure its in the correct folder. U can find it easily by browsing to a sound file in hammer, and pasting yours in that folder. make an ambient_generic entity, tick the small radius flag, and it sould loop automatically.
then you use a trigger_auto to trigger it and it should automatically trigger the sound.
match the name's in the target and trigger field and viola.
u can fiddle with the voulme to get it just right. you dont want to drown out everything else