have you checked your bios to see if windows is recognising it?
have you put it in in the correct slot order? -
1st two sticks - slots 1 & 3
2nd two - slots 2 & 4
and (i know this may sound stupid) but is it the right way up? the logo should be facing skyward on a tower unit and to your left on a flat desktop (USUALLY although this changes sometimes)
right click on "my computer" is the extra ram showing up? if so its something in cubase. if not you should look at physically re-installing the ram.