Steps to installing memory

that new memory module IS compatible with your motherboard?

anyway:
remember that static discharge WILL fry your new memory module
so before handling it, heres what you do

unplug your system

open the casing

TOuch any metal UNPAINTED part (like the chassis or the metal brackets of the expansion cards)
*this makes you static free cause the static in your body will discharge there (something like that)

flip the locks on the existing module and pull it out (vertically)

then position your new one.. check if the notch is aligned or it will bot go all the way in

push it downwards firmly but gently

if done correctly it will lock on by itself in place

you might want to go here for extra reference

http://www.pcguide.com/proc/physinst/ram-c.html