DNGFlatField in Lightroom 4, how to use/run it

[uhligfd]

I have LR 4.2 now and the "corner fix" plugin DNGFlatField from Adobe.

I can maneuver to it and then what?

How do I fix one 28mm rangefinder lens pic according to my comparison/calibration 'blue sky shots' taken with that same lens using DNGFlatField?

If I enter DNGFlatField after opening one such pic in LR, it asks me for a minimum of two pics to compare. How do I get two, i.e., another (the comparison pic) into DNGFlatField?

Is there any 'Help' or 'workflow' or 'How to use' information out there anywhere?

I am dumbfounded since I am dumb, or so it appears.