What is the 'progress' that you are trying to report? That code starts an application called x264.EXE and waits for it to finish. There is nothing in that code that suggests that any 'progress' is occurring or that there is anything to report.
I would recommend that you don't worry about a progress bar for the momemnt, but simply create a text box and display the numbers that you intend to use to indicate progress, whatever they may be.