Add-ins are used to import VBA code behind the scenes without requiring the user to copy/paste the modules into each excel file they use. Unfortunately, using the .xla add-in file is probably your best option. I would ask them to make sure that their I:\ drive is mapped to the appropriate folder.
They can do this by opening a folder, clicking tools->Map Network Drive and following the instructions from there. Their I:\ drive may have been disconnected and simply requires re-mapping to the folder that contains hstbar.xla.