Well then you have quite an empty dropbox account. So if you don't have any content in there its hard to sync anything to your device. Maybe you should install the client to your PC, this should at least create you a Public folder when it inits first time. You can also create a new folder to the root and sync that.

If you have stuff in lets say "Photos", just go to settings and select photos as the sync path and start the sync. If you want to upload from your device youll have to move some stuff after the first sync into /home/user/MyDocs/DropN900/Photos. If you want stuff from your PC into your N900, install the PC client and drop stuff into the Photos and then sync on your device.