WordPress.com and the software you download from WordPress.org are two entirely different things. Because of the way WordPress.com’s technical infrastructure is designed, we are not able to support uploading custom WordPress themes and plugin on WordPress.com. WordPress.com is brought to you by some of the same folks who work on WordPress, the Open Source blogging software. WordPress.com utilizes the same WordPress software which you can download at WordPress.org. For more info, please see this post.