Your title and the post seem slightly contradictory. If I understand correctly, you have the X-Plane USB, and you own the DVDs.
If your computer does not have a disc reader, and you have the DVDs, then you will either have to purchase an external disc reader or borrow/rent one. The DVDs have the global scenery on them, and depending on the amount of scenery installed, there are roughly 60GB of files that would need to be installed. The USB key only unlocks the demo for the demo area without a time limit or other restraints, but the global scenery will still not be installed. Thus, you should do the aforementioned if able.