Hi,
From running 'man ls' in Terminal:
... "If the file or directory has extended attributes, the permissions field printed by the -l option is followed by a '@' character. Otherwise, if the file or directory has extended security information, the permissions field printed by the -l option is followed by a '+' character." ...
I recommend
http://www.bruji.com/bwana/ for efficient access to you man pages in Safari.
I don't understand how the Finder can't find the file anymore on you machine; extended attributes are very common additions to files saved by many programs these days, since Tiger.
Oddly, Wikipedia has good information on it, including links to Apple's command-line tools (bottom of page). If you remove the attribute(s) they will probably just reappear next time you save the file from Excel.
Larry