Illuminate keyboard using Scroll Lock key
The idea is generally speaking possible.
You must know where exactly the scroll lock led is connected. There are 2 possible variants - between the +5 and some output and between ground and some output. 
You should use a transistor in common emitter schematic.
Learn to draw schematics. Now you are like a stone age man writing a letter using pictures. It is very hard for you and very hard for others. Learn the alphabet of the electronics - schematics.