editing format im.incident.subform.sla

I need to edit format im.incident.subform.sla

When I go to format designer and go to the form, nothing happens when I click Design.  I've never come across a form that doesn't open in design mode before. 

Anyone know why that might be or how I can edit it?  I also tried going to form details but renumbering the lines to move things where I wanted did not work either.