Well, the main problem here is that an Excel spreadsheet is not designed to go on the web. It's like asking, "How do I import this web page into Excel?"
That being said, it's not impossible, but you need to be more clear about what you want to do. Do you simply want someone to be able to download the Excel file? In that case all you need to do in iWeb is select the image or text that you want the person to click in order to download the file, go to your Inspector palette, select the far right "Link" tab, choose "Link To A File," and select your Excel file.
If you want the Excel file to be displayed as a separate web page, you can do "Save As Web Page..." in Excel. (This will result in really bad html code because of the first thing I mentioned in this post.) You won't be able to import this html file into iWeb, though, because iWeb is too basic. However, you can upload this Excel-generated html file onto your server separately from iWeb and link to it within iWeb just like you'd create a link to any other site.
Finally, if you want the Excel spreadsheet to appear like an image in your web page, probably the easiest way would be to do a Print to PDF within Excel, open the resulting PDF in Preview, save as a JPG or GIF, and import that image file into iWeb.
I think those are your options.