Greetings! I can tell you as an owner of the ASUS G75VX that the two additional memory slots are in fact under the keyboard.
To access these, you will need to remove most of the "A" screws on the bottom of the laptop including those that are accessed under the panel that hides the hard drives and RAM. You will also need to remove the laser disc tray (one screw and it slides out) because there are 3 smaller screws under there that help hold the keyboard down.
There are multiple "tabs" that hold the keyboard down and as you pop them apart you will see where you might have missed a screw as the keyboard will appear to "hang" from that point.
There is a youtube video that NEARLY covers everything you need step by step to do to flip up the keyboard to access the RAM. It is in German, but that is not important as it shows you ALMOST all of the screws. What the video WONT show you is the removal of the 3 screws that are under the CD-Tray along the edge of the laptop.
http/www.youtube.com/watch?v=8xaC1yKGS4s
On a side note, I found the RAM kit below works best for the G75VX.
http/www.crucial.com/store/mpartspecs.aspx?mtbpoid=4F53FFA3A5CA7304
Hope this helps.
-Rathamus
Founder of the World of Warcraft Public Ventrilo