If you drag an Excel file onto the Numbers icon, it doesn't open in Numbers?
The error message you're seeing is because your iMac's been upgraded to the Lion OS, and the Lion OS doesn't support PPC applications.
You've got a couple of choices:
1. You can spend the $100 or so to upgrade to Microsoft Office for Mac 2011, which, by the way, includes a free update to the next version of Office for Mac.
2. You can convert all of the Excel files to Numbers. Be aware, though, that there may be some formatting changes made during the conversion.
I'm not at my Mac right now, but there is a way to associate files with a given application. It involves right-clicking the file, choosing Get Info, then changing something in the resulting window. Unfortunately, I can't provide the exact wording just now.
I hope this helps.