The easy way to do it, and the way I have my BL3s connected to my PowerMac, is to use a Beoport. This allows you to control the music by remote, also allows you to link the computer music to any other B&O you have in the house and has a Powerlink socket so that you simply connect that to your BL2 and then connect the BL3s to that.
Otherwise it will be making up a lead - stereo minijack to Powerlink. I take it the problem is the switching voltage.