How can I unlock dual dimension?

HI,

We are on Wildfire 5.0.  At this moment we work with dimensions in millimeters. I would like to bring up this dimension in inches. In the config.pro, the parameter pro_unit_length is set to unit_mm. It's what we want for most of the time.  But sometime, I want to insert the equivalent dimension in inch in my model or my drawing (ex: 50.8mm = 2").

I try to edit the dimension but the DUAL DIMENSION is lock (see picture below)

What is the parameter I need to unlock in the config.pro to allow this option?

Thanks for your help