The TOP slot is a single load slot which should be occupied by your nav disk if you have one. If you want to put a CD in to play (or MP3 CD, or DVD-A), you must press the little button to the left of the LOWER slot that looks like a bunch of CDs stacked on top of eachother. You will then hear the mechanism prepare itself to accept the first CD (it is a 6 cd changer) and when it is ready it will say "INSERT DISC" or some such thing and you insert your first disc. Once it is successfully loaded, you repeat this process for as many disks as you have, up to 5 more times.