It looks like Impress doesn't have the ability to insert an object as an icon, and so doesn't know what to do with objects that are represented as icons (instead of looking like Excel, in this case).
Options I can think of:
- Getting someone with Office to save the object as a file and send it to you
- Getting the document's author to insert it the regular way instead of as an icon, since Impress would then give you "Edit" and "Save Copy as..." choices
- Using the free PowerPoint Viewer, if you're running Windows (assuming it handles these objects correctly—can someone confirm?)