In your home directory, on a desktop, you can drag the file into the Dropbox folder, or a sub-folder of your choice. Make one, called Spreadsheet for instance. However, if you have no desktop/laptop, the Dropbox App needs used or an integration, so you can select a file to copy into it.
The option in Numbers for instance, gives any imported file in there a chance to be saved out via WebDav, so you would follow th e protocol in part by putting in your DropBox URL, login and password. Then you should be able to save it in there.
Does that work, you did not really point out details on what is going on in your situation.