You need special permission to be able to upload a torrent.
First of all you need to contact a mod and tell them what you have to upload (bearing in mind things like rules over availability and quality). If hey agree that it's wanted, they will upgrade your status from member to uploader and a new "upload" button will appear on the index page.
If that goes through, you create a torrent with your client (Utorrent or whatever), upload the torrent file (not the full file) and then re-downlod the new torrent file provided by mousebits. add it to you client and it will then seed to other members.

If it's a small (under 200mb) and falls within the rules, you can post to a free fileshare site (like mediafire) and post the link to it in the forums with a request that an existing uploader make the torrent.
It's quite usual to do the latter first and then become an uploader later when they have had a chance to sample things (an make sure they are not just rips from Youtube etc).
Hope this helps.