Whopps, forgot the password :) (gwmi -Namespace root\dcim\sysman -Class DCIM_BIOSService).SetBIOSAttributes($null,$null,"Wake-On-LAN","4","Password") This one works!
Hi! Just discovered this beautiful tool OMCI that would make it possible to enable Wake on lan and other stuff in the BIOS with WMI. It works great except one command! The command that actually change settings in the BIOS. 
Dell OMCI 8.1: Can't enable Wake on Lan with WMI.