References

If you have created references between workbooks in your project, you must do one of three things to ensure that the references are maintained correctly. Either the relative paths between the referencing and the referenced workbooks must be maintained (you must open the referenced workbook(s) prior to opening the referencing workbook(s)), or you must have the AddIn in the AddInPath. The AddinPath is a registry key value that indicates the search path for Addins.

HKEY_CURRENT_USER\Software\Microsoft\Office\8.0\Excel\_
    Microsoft Excel\AddIn Path.

The value for this entry is of type string, with a format like:

c:\msoffice\excel\library; c:\msoffice\excel\library\solver

The ';' character delimits each directory in the path.

By default, Office 97 setup sets the AddIn path to include the Library subdirectory of the Office subdirectory that contains Excel.exe. So you may want to recommend that users copy your add-in into that directory (Application.LibraryPath).

When the workbooks are closed, you must close them in the opposite order. Microsoft Excel does not allow a referenced workbook to be closed while the referencing workbook is still open. The referencing workbook must be closed first, followed by the workbooks that are referenced. This applies to both add-ins and normal workbooks.