I am sorry, but advertising is not allowed on free hosted WordPress.com blogs. The software is tripped the code out because it's not allowed. The only exceptions are for high traffic blogs that qualify for and are accepted into the Ad Control program, and very high traffic blogs that qualify and are accepted into the paid VIP hosting program.
If you require an ecommerce site, advertising and/or affiliate links on your blog, you can hire a web host and get a free software install from http://wordpress.org In that case, please see the information at the link below to learn about the differences between free hosted WordPress.com blogs and WordPress.org software installs for self hosting. http://support.wordpress.com/com-vs-org/