I saw this in the help menu and didn't know if it applied to you but
thought it was worth posting:
The shared workbook can be opened only as read-only.
Check whether the workbook is an Excel 95 shared list When you open a
Microsoft Excel 95 shared list in Microsoft Excel 2004, you see a
message that shared files from Microsoft Excel 95 must be opened
read-only. To convert the file to a Microsoft Excel 2004 shared
workbook, click Save As on the File menu, click Excel Workbook on the
Format pop-up menu, and type a new name in the Save As box. You can
then share the workbook.
Check whether the read-only message is a recommendation from another
user Another user may have recommended opening the file as read-only.
In this case, you can choose to open the file for editing and saving by
clicking No in response to the message suggesting that the file should
be opened as read-only.
Use Save As to edit data Another user may have saved the workbook
file as read-only. To edit and save the data in the file, you can use
the Save As command on the File menu to save the shared workbook with a
new name so that it can be edited.