This post helped me ... I was successful in getting numlockx installed in Mint 8 but I couldn't do it through the Synaptic package manager because all of the repositories are long since dead ...
I won't go into the gory details of how I found everything, but here was the solution:
I downloaded: numlockx_1.1-10build1_i386.deb
to my USB thumb drive at /media/artwork/linux/general
"artwork" is the label of my Thumbdrive, and /linux/general is the path to my working folder.
From a Terminal session:
mint@mint ~ $ sudo -i
mint ~ # dpkg -i /media/artwork/linux/general/numlockx_1.1-10_i386.deb
This installed numlockx thus enabling the on, off, and toggle options for the numlockx command!
Blessings in abundance, all the best, & ENJOY!
Art in Carlisle, PA USA