I have tried a number of different settings from using a self-signed certificate, to not requiring a certificate at all. nothing changes.
The VSTO add-in installs without a problem.
Also, if I open the file from a different location, then save it to the Sky Drive, everything works--until I close and reopen the application (A major security problem??)
Someting undocumented has changed between 2010 and 2013.
JC