You can't upload games onto the site, you have to get webspace (your ISP should provide you with some). When you've got your game on the Internet, go to the Downloads section, and on the right side of the page there is a 'Submit your downloads here' link, or similar.
Although interestingly, when TDC was first concieved in this format, you did have the ability to upload games to the DC server. Unfortunately this got clogged up pretty quickly with..well... crap. So it was removed.