I am not with Laminar Research; just a flight simmer.
To run X-Plane 11 the DVD set requires disc 1 to be inserted into the dvd drive basically at all times. Disc 1 will not make a direct copy to an alternative medium. I know, having tried it.
Others who have the same problem as you have raised the same problem in this forum and have been advised to create an ISO disc from disc 1. Do a search back through earlier forum questions (some months back)
For me personally I also have a disc set but my dilemma is trying to preserve the integrity of disc 1 without having to insert the disc. XP10 has an alternative arrangement of using a USB dongle. There is no dongle for XP11. At this stage I have not created an ISO replacement so I can't advise on how successful it would or will be.
On the basis you have a USB connection you can create a substitute ISO disc onto a USB drive instead. I would suggest you consult Dr Google on how to do it. I found several links. Which method will give the best result is a matter of trial and error. If you have a DVD drive on your old Mac you may be successful in creating the USB ISO otherwise you will need the assistance from a mate's computer.
As officially advise by Laminar Research there is no product key available for X-Plane for the DVD set. You have to use disc 1 of the DVD set. If your DVD set is the same as mine, not a Steam version, from Aerosoft, the registration key supplied in the package is a sales pitch ploy or "con" to encourage you to purchase additional products from them at a discount price.
Hope this helps and welcome to X-Plane.