For New Projects:
If you're creating a new project, the last step in the process is uploading your files. Here you can drag and drop files into the blue area on the screen, or you can click the blue area or the text link directly below this area and browse/upload the traditional way. Both work and also allow you to add multiple files at once.
For Existing Projects:
If you already have an exiting project, but want to add more files, you can click the "UPLOAD FILES" button on the right side bar. This will give you the same modal window as you've seen before and you can upload more files to your project from here.