Links to Add-in on network

We have an addin sitting out on the network so that users can share it
and it can be updated without having to redistribute to all the users.
The mapped drive is U:\. We open the worksheet from Access or we open
it from windows explorer. When we open it from Access the sheet opens
fine and there is no problem. When we open it from Windows Explorer the
add-in cannot be found and the formula starts to show the UNC link or
the Mapped drive link in the formula itself even though the add in is in
the directory that it's pointing too. The add-in function is not found
at this point. Are there any ghost links in the registry i can hunt
down or any reason that the add in comes up as not found even though it
exists in the network drive?