I can't figure out why the developers of dropbox have to over-complicate simple things like "selecting all files". I finally found directions how to do it and it's just plain stupid and doesn't always work.
"Selecting multiple "Selecting multiple files
Click on the file row to the right of the file's name to select it. (Clicking on the file name or icon will open the file as a preview.)
Hold the Shift key and then click on or use the up/down arrows on your keyboard to select another file in the file browser."
Why can't you just make it "ctrl-a" like everybody else in the world? Why make technology harder to use for NO REASON.