I think its because VS cant see the SF Addin. If the path to the Addin is user based like C:\Users\YourUserName\AppData\Roaming\Microsoft\MSEnvShared\AddIns on Vista
or
C:\Documents and Settings\Username\Application Data\Microsoft\MSEnvShared\AddIns on XP
then VS cant see the addin on the other users.
/Teddy