It will be somewhere in the top right hand corner of the keyboard, if it's labelled in a different colour than the normal keys then you'll need to press the and hold the FN key (bottom right of keyboard) while you press the num lock key
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
Thing is there is no such thing as "a dell laptop"... dell have made loads of different laptops over the years and a single instruction is not going to work for all of them.
It's a bit like asking where the light switch is on a ford car and expecting any answers given to be correct for a ford model t or a brand new focus.