It can't be done at wordpress.COM directly since wordpress does not allow uploading of excel spreadsheets. Also, we can't put data such as spreadsheets and such into the wordpress database. That would be a security risk since people can use excel macros for evil.
Your choices are:
1. Upload them to a third-party file hosting service such as fileden or box.net and link to them from your blog.
2. Upload them to Scribd and then use the process outlined in this support document to display them on your blog: http://en.support.wordpress.com/scribd/
The problem with displaying spreadsheets on a blog is that most spreadsheets are too wide for the content area which means part of them will end up being cut off.