OK, I am a little dense here.
What file location do you open up to actually change the sensitivity value in?
I see a description of it as mentioned in the "" file, but where do you actually make the change itself.
You can find the setting in the file user[dot]bnd. Just open the file in Notepad and edit it, by default it has the line "mouse_sensitivity_y_scale 1" so just change the number to whatever you wish the y-axis factor to be.
edit: Having some problems posting the filename.. Hope you find it :P