I checked under keyboard and there is no option to adjust "Mouse Keys". I turned it on under "Universal Access", but there was no option to adjust sensitivity or acceleration there. Under "Mouse and Touchpad", there were two tabs, one for a touchpad and one for a mouse, neither of which affected "Mouse Keys". Anybody have any idea where the devs decided to put the options in precise?
This is more of a workaround than an answer to your question. You can use xkbset to adjust the speed of the cursor when using the mouse keys feature. The idea came from this post:
The setting I found most comfortable was:
Hope that helps!