At the top there is a bar the says Downloads. To upload a file you simply need to click My Files, then press Add a Download.
The rest of it should be pretty simple, add the name, the file, any description you want, put it in the category you like and then hit upload.
From there it will be up to the Mods and Admins to approve uploaded files.
Please feel free to give it a shot and let us know of any further issues.