OT Master...I'm missing something basic

This is embarrassing. I've had DTL's OTMaster for quite a while, but I've only just played with it. 
Today I wanted to edit a GSUB entry...a ligature entry points to the wrong glyph. So I open the gpos/gsub table viewer, and I can see the problem, but the edit menu is greyed out...and remains so in any of the views I can find.

This persists with different fonts as well.

What am I missing?