FreeCAD uses it own python installation
serial is installed but not found
If you type in the python console window
an error will occur.
your package is installed to
c:\program files (x86)\python35-32\lib\site-packages\
there is a pyserial-3.1a0-py3.5.egg and a serial-subdirectory
but it should be installed
your FreeCAD directory\bin\Lib\site-packages
May be it helps to copy the directory and the egg file to this position.