After you select a category, you can upload your file. Currently, you may upload up to 5 files with a maximum file size of 64MB each. This is separate from adding files to the description -- Use the top Attach Files button for your addon's files, and use the one under the description box for images of your addon!
If you have an addon file that is larger than 64MB, you can use software such as 7-Zip or WinRAR to create a split archive of it. Just tell the software to split the archive into 64MB chunks. If you have a 90MB file called mod.pk3, you can use this technique to split it -- the resulting archive chunks will be mod.001 (64MB) and mod.002 (26MB). The bigger your file is, the more chunks you'll need. You can add all files required by your addon in this way if you like. All the end user will have to do, assuming they have WinRAR or 7-Zip, is put all the archive chunks in the same directory, then open any of the chunks to view the files and extract them.
If your file is more than 320MB total using this method, please contact a judge and we'll see what we can do to accommodate.