Hi,
Add-ins are in a users profile, so if you have logged in with the Administrator account and installed add-ins, and then log in as a different user, that user will need to install the add-ins too - perhaps that's what's happened in your case?
Add-ins are stored in the profile under:
%APPDATA%\ESRI\arcgis explorer\AddIns
e.g. in Windows 7 for an account called SHELLY that would be:
C:\Users\SHELLY\AppData\Roaming\ESRI\arcgis explorer\AddIns