Can't Find Original Serial Number [for Lightroom 5]

I bought a macbook pro without realising that my adobe software would no longer work on the new platform. I can't even download the adobe cs6 suite from the dvd, however I have a lightroom 5 instalation disc which downloaded fine. I put the serial number in but it then said this was an upgrade and to put in the original serial number, but I'm sure I chucked out that disc years ago. How do I find my original serial number? The upgrade number appears in my abobe account but not the original