Under 64-bit Windows, the installer program defaults the installer directory to "C:\Program Files (x86)\" instead of "C:\Program Files\" where SketchUp is usually installed.
The solution is probably some combination of the InnoSetup flags for [Setup] ArchitecturesInstallIn64BitMode and [Setup] ArchitecturesAllowed. Or else do some registry checking (difficult) to determine where SketchUp is actually installed.
The user was mistaken. Upon further testing the user reported that the installer works fine under 64-bit Windows. "C:\Program Files (x86)\" is actually the correct directory.