leppy600m over at insanelymac forums put an awesome howto together for d420 laptops.

this worked for me perfectly except for trackpad scrolling.

http://www.insanelymac.com/forum/ind...ic=178411&st=0

you also should read the d420 install bible here to learn sleep function:
http://www.insanelymac.com/forum/ind...c=110053&st=20
(tip is to set a password in bios after installing a kext from dell laptop post installer 1.3.1

good luck