Mine is MS Office 2004, because it has taken Microsoft this long to come up with an update for the Mac (I don't yet have the new one). On mine, you hit "Print" and a menu comes up showing your printer and what pages you want to print. At the lower left is a button saying "pdf." Hit that and you can save your Excel spreadsheet as a .pdf file. That's what you are going to upload. On this site, go to "My Account" and then "Change my Profile." At the bottom is a button you click to choose the file to upload.
I haven't tried this yet, but it looks pretty easy even to this comparative luddite, once I found the pdf thing on the Print menu!