Why is my number pad not working?

Dell seems to have more keyboard issues, at least in my experience with the E6540. Granted, my E6440 was fine with the original keyboard before I did the keyboard/palmrest swap, but it does happen on these more than other brands like HP and Lenovo. That said, check if the num lock key is dead or just disabled (lock with 9 as the “key”)—sometimes you get lucky.

The DP/N is 564JN (US English Backlit) or M8F00 (US English non-backlit) if you need to replace it. With this machine, you can often find the backlit version for a few bucks more so I'd encourage you to fix that issue if yours is too cheap not to include it; it's a ~$5-10 upgrade. In addition to the fact that the upgrade is so cheap when it fails to fix a configuration mistake, it can be converted by swapping the cable, and the BIOS will take it right away; no coding is needed. If I had to do it again on a machine I know is upgrade-ready like these, I'd do it again.

At least on my E6440 when I ditched the E5440 keyboard (it was a low end model with the LVDS 768p LCD and no webcam as well), it took it right out of the gate, even lacking the buttons; I later added them via a used palmrest in better condition to complete the upgrade. Since I already put an HD+ screen on the unit, what harm am I doing by upgrading the keyboard since I drifted from the original spec when I also had to replace the keyboard AND I got a good deal on the better parts?

That being said: This is a possible variable on the newer E5/E7 series machines where you may not be able to upgrade it without adding parts to the motherboard (but it's also highly likely it will work since Dell probably uses one board to save costs and omits it in the initial configuration). I would probably make sure it has it, but if the price was agreeable and I don't see 2 motherboard options I'd upgrade it if the keyboard ever failed, no question.