Thanks for the reply, but I've been down that road.
Grib files downloaded by AirMail are file type .msg, hence they aren't viewed by OpenCPN grib plug-in (at least in my case).
Furthermore, when I do import
a grib file into the Grib plugin popup box, to which you refer, the file name is placed in the upper left quadrant of the popup and is greyed out.
I am anticipating that once I get the download source linked into the OpenCPN grib plugin, I should see all available grib files, from which I select one and it gets overlayed. The number of steps required to locate the required AirMail download is complex and far from user friendly.
So I am thinking that I am missing a large piece of the puzzle.