Sorry should have specified in post, Office 2010 (including Outlook) is installed (natively?) as part of the default vdi image (gold build).
Trying to add the add-in if possible with a Thin-app package to just a selection of users.
I did contact the vendor who provided this info on Reminder Manager "Reminder Manager is a COM dll that runs in-process with Outlook, and must be registered both as a COM dll and as an Outlook addin in the Windows Registry in order to be able to be started by Outlook". This may help in providing an asnswer as to weather the add-in will work via a package or needs to installed directly.