Head to your project overview.
Click the grey "Files" sub-tab.
you will see a number of file groups, to view the files click the blue "View" button.
you will have two file groups that you are able to upload files to. You can upload files by clicking the blue "upload" button and either drag and drop the files into the upload box, alternatively, you can click the blue "add file" button.
Once you have added your files, click the blue "Start Upload" button and check the status of the file, if the status is 100% then your file is uploaded and you can click the blue "Close" button.