how do I engage number lock?

Did you press the Function key plus F4?. Did you check Help and your manual? Make sure the F Lock key, if there is any, is not toggled on or off. Here's an article from MS about that: Click_here.

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I had this problem too.  I spoke to Dell Support.  Apparently, Fn-F4 turns on the number lock light regardless of whether or not the number lock feature is enabled.

Here is how you enable the number lock feature:

1)  While booting up press F2 repeatedly

2)  After the Dell logo appears, you will be diverted to a Settings menu

3)  Expand "POST behavior"

4)  Select "Keypad (Embedded)"

5)  Press the "By Num Lock" button

6)  Press “Apply”

7)  Press “Exit”