Error: Cannot find module 'serialport'. node.js, Arduino Yun
I solved the problem by following these steps:

Make a folder node_modules in mnt/sda1/

cd /mnt/sda1

wget http://wiki.linino.org/lib/exe/fetch.php?media=wiki:serialport-1.6.1_mips-25-ubuntu.tar.gz -O serialport-1.6.1_mips-25-ubuntu.tar.gz

run $ tar xvf serialport-1.6.1_mips-25-ubuntu.tar.gz -C /mnt/sda1/node_modules