Nothing Happens when I click Optistruct

I am trying to use Optistruct for Topology Optimization.
 

I imported a geometry in a STEP file, meshed it, gave it material properties, loads & constraints - However when I click to run Optistruct, nothing seems to process, and no screen appears. 
I think I am supposed to at least get an .out file to at see the status of my work. When I click 'view .out' - it says 'out file does not exist' on the bottom left corner of the screen

<?xml version="1.0" encoding="UTF-8"?>