I haven't used that old version of InstallShield Express in many years, so
my recollections are rather vague, but what I do in later versions of
InstallShield is to set the registry keys I want added for the addin
registration (LoadBehavior, FriendlyName, etc.) to the Users hive of the
registry in the registry settings pane and set the installer to let the user
choose current user or all users. The MSI generated by InstallShield then
puts the settings wherever is appropriate.
In that case I would probably use a class module to handle the
IDTExtensibility events instead of a designer class to let the registry
settings be guided solely by the MSI installer. Otherwise you can end up
with registration in both HKCU and HKLM.
The designer sets the registration to HKCU always, so I'm not sure why you
are getting registration in another hive unless you have something for that
set in your installer project.