Program folders might be hidden from standard users. Personally I created a shortcut to the 32-bit apps folder. Open a folder & type this in the address bar:
C:\Program Files (x86)

You will find the Mozilla Firefox folder inside.

You can also use
\Program Files (x86)
to type into the desktop shortcut creator & it will work.

To do it the way you are trying to do it, I also had same issue. But, you can use the Browse button to find the program in question.
Computer -> Local Disk (C:) -> Program FIles (x86) -> Mozilla Firefox -> firefox.exe

I suspect the problem with typing in the address of the file in the above process is the path references. There's two ways of specifying paths in windows, 'relative' path & 'absolute' path.
Paths using \ need to be escaped using \\.
This works:
\Program Files (x86)\\Mozilla Firefox\\firefox.exe

This also works:
\Program Files (x86)/Mozilla Firefox/firefox.exe

Notice I also needed to specify the firefox extension.

I learnt a little about this when reading ArneBab's mini LW wiki about creating portable LimeWires. And his references helped me to create the LW Security package so LW references it properly.