Hi, do you have the Dropbox app installed on your Mac? If so, you should have a folder called "Dropbox" on the computer which keeps a copy of your Dropbox files. You can open this folder by clicking the Dropbox icon in your computer's menu bar, then clicking the folder icon in the top-right corner of the window.
If you're trying to upload files to a website, you can usually drag the file from the Dropbox folder onto the "upload" or "select files" button of the website, the same way you would if it was a file outside of your Dropbox folder.
