I tried to upload a zip file, but nothing happened? How does this process work for projects / zip files?

You should be able to zip your entire Xcode project directory tree and upload it using the Converter – Project tab.
Since the conversion may take a long time for large files, you may consider excluding anything optional from your upload.

Examples:

• Source control directories (.git, .svn etc)
• Large files - data / resource.

 You can simply add these files back to your project directory tree after conversion.