Your Dropbox folder should just be in your home directory unless you otherwise specified.
Just be sure you don't run Dropbox sync at the same time as Calibre! That will seriously mess things up. Just pause sync on Dropbox before you open Calibre.
I use mine with Copy, a service similar to Dropbox. I had a bunch of glitches with books getting missing, etc., until I realized I was running Copy sync at the same time as Calibre. Stopped doing that, and all is sweetness and light.
