Please do a forum search for “plugin” and “adsense” and you will quickly see the reason why. WordPress.com does not allow the bloggers to install plugins or adsense. We are on a shared blogging platform, which means that the software for your blog, including the template, is shared and if you installed them on your blog they would go on everybody’s. For security reasons staff have not allowed this.
If you find you MUST have these options, you can go to WordPress.org, where you can download the WordPress software and install it in your own webspace.