mmmmmm, in fact you should have had a mtl.temp file as this is the one that doesn't exist here and threw the error. Do you have the option "export materials" checked in the plugin config? (according to my code, I forgot to skip renaming the mtl.temp into mtl in case this option is not checked... it would be easy fix but it'll be nice you first confirm this is the reason of your error).
Desktop: Ubuntu 13.04 x64 - i7-3770K @ 3.5GHz - 32GB DDR3 - GTX670 2048MB
Laptop: Linux Mint 11 x64 - i7-2860QM @ 2.5GHz - 16GB DDR3 - Quadro 3000M 2GB
Software: NVidia 319.12 - Cuda 4.2.9 - Blender 2.66a