Hi,
The Activation Code is not on the DVD (as data).With Cubase LE you got a paper (or maybe it was a sticker on the DVD) with a Download Access Code or a Serial Number (in the past). Use this in MySteinberg, to get the Activation Code.
You are right, you need to Activate your old Cubase LE first, to be able to get the upgrade to the current Cubase LE 10.5.