1) Did you check C:\Documents and Settings\\Application Data\Microsoft\AddIns and see if the file is there? If it is, you could delete it from there.
2) Try these steps:
• Try searching your C drive for all .xla or .xlam files and see if you can find the one that looks like the add-in you're trying to delete.
• If you find it, delete it (let it go to recycle bin though just in case something goes wrong and you want to get it back quickly).
• Once you delete it, start Excel. You may get message about add-in being missing, and if so say yes when prompted if you want to remove add-in from list.
• If not prompted, go to tools add-ins and check the box for the add-in you are trying to delete. Excel won't be able to find it, then it should prompt you as to whether you want to remove it from list. Say yes.