Scratch

If you mean a project made in a mod, that's against the Source Code License.

Well, the only thing you can do is make a forum topic about one or put it on the wiki.

I'm confused.
You don't make mods in Scratch, you make them in Squeak.

This website is meant for uploading Scratch projects, not mods, and unless you clarify what you mean, that's the best answer I can give.

Oh, that's a whole different story.
If you're using Windows, put the .exe file (probably started as Scratch.exe or Squeak.exe, change it to the name of your mod) in a zipped folder.

Then, you add the image file (don't add the .changes file).
Then, add all of the .dll files (ScratchPlugin, UnicodePlugin and WeDoPlugin).
After that, add all of the other files you may need, and then upload the zipped folder to the website.