When I open the Add-In Manager window from Tools\Options\Other\Advanced, it
has a button labeled "Install." Clicking on it presents a file open window
on the ADDINS folder of my Office installation folders, showing the .ecf
files available.

OK, I write the code. Someone else writes the installation script. I have
one environment where the user has to come to this window and use this
"Install" button to manually install the ecf file for my add-in. In all
other environments, my add-in is automatically installed by the installation
kit that installs the containing product.

I have checked, and all of the appropriate entries are being added to the
HKLM\SOFTWARE\Microsoft\Exchange\Client\Extension key.

I have been asked to find out why. But I don't know exactly what it means to
"Install" the ecf file. Can someone enlighten me?

Thank you.

Dave Smith