There could be any number of reasons for that. There could be an exception
in your OnConnection() event handler, you might not be deploying all of your
application's dependencies, the user might not have the correct PIA's or
Framework version installed.
In addition, if the user installed the Framework after installing their
version of Office they might need to have KB9008002 installed. That installs
extensibility.dll, stdole.dll and the registry lockback key. If either
extensibility.dll or stdole.dll aren't installed that would cause such an
error on addin load.
There are other possibilities also, there just isn't enough information to
really tell. You might have a strong naming problem if you used strong
naming. If you're using a shim it might not be set up correctly. There are
many other reasons for the error you're getting, you're just going to have
to eliminate the obvious possibilities first and go from there.