I'm confused on what (x86) is..
I take you you mean c:\program files (x86)
on 64 bit windows installations
there will be 2 program files folders
c:\program files (x86)
x86 = 32 bit installations
on 32 bit windows installations there is only 1 default program files folder
so find your diablo installation in either one if you need to.
btw: i think loading mpq files on the thumbdrive via symbolic links is just troublesome in its implementation, you should simply make a copy of your whole diablo installation to a thumbdrive. then run the game off the thumbdrive directly. no fuss/with much easier to understand instructions.
if you want a launcher, then open up notepad. type in
start "" "h:\diablo iii\Diablo III Launcher.exe"
(where h:\diablo iii) is the usb drive\folder than contains the d3 install.
then save it on your desktop as d3usb.cmd
double click that to play. you can pin it to taskbar/start menu if you want as well